[FIR] Cleanup: drop unused ExpectActualMemberDiff

- It became unused after cc70f78027
- And it's not going to become used in the nearest future, since in K2,
  we won't have "member scopes" check on frontend (for now)

core/compiler.common/src/org/jetbrains/kotlin/resolve/multiplatform/ExpectActualMemberDiff.kt

@@ -1,85 +0,0 @@
-/*
- * Copyright 2010-2023 JetBrains s.r.o. and Kotlin Programming Language contributors.
- * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
- */
-
-package org.jetbrains.kotlin.resolve.multiplatform
-
-// This class will be later used by K2. That's why it's placed in compiler.common module
-data class ExpectActualMemberDiff<out M, out C>(val kind: Kind, val actualMember: M, val expectClass: C) {
-    /**
-     * Diff kinds that are legal for fake-overrides in final `expect class`, but illegal for non-final `expect class`
-     *
-     * Also see: [toMemberDiffKind]
-     */
-    enum class Kind(val rawMessage: String) {
-        ReturnTypeChangedInOverride(
-            "{0}: the return type of this member must be the same in the expect class and the actual class. " +
-                    "This error happens because the expect class ''{1}'' is non-final"
-        ),
-        ModalityChangedInOverride(
-            "{0}: the modality of this member must be the same in the expect class and the actual class. " +
-                    "This error happens because the expect class ''{1}'' is non-final"
-        ),
-        VisibilityChangedInOverride(
-            "{0}: the visibility of this member must be the same in the expect class and the actual class. " +
-                    "This error happens because the expect class ''{1}'' is non-final"
-        ),
-        SetterVisibilityChangedInOverride(
-            "{0}: the setter visibility of this member must be the same in the expect class and the actual class. " +
-                    "This error happens because the expect class ''{1}'' is non-final"
-        ),
-        ParameterNameChangedInOverride(
-            "{0}: the parameter names of this member must be the same in the expect class and the actual class. " +
-                    "This error happens because the expect class ''{1}'' is non-final"
-        ),
-        PropertyKindChangedInOverride(
-            "{0}: the property kind (val vs var) of this member must be the same in the expect class and the actual class. " +
-                    "This error happens because the expect class ''{1}'' is non-final"
-        ),
-        LateinitChangedInOverride(
-            "{0}: the property modifiers (lateinit) of this member must be the same in the expect class and the actual class. " +
-                    "This error happens because the expect class ''{1}'' is non-final"
-        ),
-        VarargChangedInOverride(
-            "{0}: the parameter modifiers (vararg) of this member must be the same in the expect class and the actual class. " +
-                    "This error happens because the expect class ''{1}'' is non-final"
-        ),
-        TypeParameterNamesChangedInOverride(
-            "{0}: the type parameter names of this member must be the same in the expect class and the actual class. " +
-                    "This error happens because the expect class ''{1}'' is non-final"
-        ),
-        Unknown(
-            "{0}: normally, this error should never happen. Please report to https://kotl.in/issue. " +
-                    "This error happens because the expect class ''{1}'' is non-final"
-        )
-    }
-}
-
-fun ExpectActualCheckingCompatibility.Incompatible<*>.toMemberDiffKind(): ExpectActualMemberDiff.Kind? = when (this) {
-    ExpectActualCheckingCompatibility.ReturnType -> ExpectActualMemberDiff.Kind.ReturnTypeChangedInOverride
-    ExpectActualCheckingCompatibility.ClassTypeParameterCount -> error("Not applicable because ExpectActualMemberDiff is about members")
-    ExpectActualCheckingCompatibility.ClassTypeParameterUpperBounds -> error("Not applicable because ExpectActualMemberDiff is about members")
-    ExpectActualCheckingCompatibility.ActualFunctionWithDefaultParameters -> null // It's not possible to add default parameters in override
-    ExpectActualCheckingCompatibility.ClassKind -> error("Not applicable because ExpectActualMemberDiff is about members")
-    ExpectActualCheckingCompatibility.ClassModifiers -> error("Not applicable because ExpectActualMemberDiff is about members")
-    is ExpectActualCheckingCompatibility.ClassScopes -> error("Not applicable because ExpectActualMemberDiff is about members")
-    ExpectActualCheckingCompatibility.EnumEntries -> error("Not applicable because ExpectActualMemberDiff is about members")
-    ExpectActualCheckingCompatibility.FunInterfaceModifier -> error("Not applicable because ExpectActualMemberDiff is about members")
-    ExpectActualCheckingCompatibility.FunctionModifiersDifferent -> null // It's not possible to override with different function modifier (suspend)
-    ExpectActualCheckingCompatibility.FunctionModifiersNotSubset -> null // It's not possible to override with different function modifier (infix, inline, operator)
-    ExpectActualCheckingCompatibility.Modality -> ExpectActualMemberDiff.Kind.ModalityChangedInOverride
-    ExpectActualCheckingCompatibility.ParameterNames -> ExpectActualMemberDiff.Kind.ParameterNameChangedInOverride
-    ExpectActualCheckingCompatibility.PropertyConstModifier -> null // const fun can't be overridden
-    ExpectActualCheckingCompatibility.PropertyKind -> ExpectActualMemberDiff.Kind.PropertyKindChangedInOverride
-    ExpectActualCheckingCompatibility.PropertyLateinitModifier -> ExpectActualMemberDiff.Kind.LateinitChangedInOverride
-    ExpectActualCheckingCompatibility.PropertySetterVisibility -> ExpectActualMemberDiff.Kind.SetterVisibilityChangedInOverride
-    ExpectActualCheckingCompatibility.Supertypes -> error("Not applicable because ExpectActualMemberDiff is about members")
-    ExpectActualCheckingCompatibility.TypeParameterNames -> ExpectActualMemberDiff.Kind.TypeParameterNamesChangedInOverride
-    ExpectActualCheckingCompatibility.TypeParameterReified -> null // inline fun can't be overridden
-    ExpectActualCheckingCompatibility.TypeParameterVariance -> null // Members are not allowed to have variance
-    ExpectActualCheckingCompatibility.ValueParameterCrossinline -> null // inline fun can't be overridden
-    ExpectActualCheckingCompatibility.ValueParameterNoinline -> null // inline fun can't be overridden
-    ExpectActualCheckingCompatibility.ValueParameterVararg -> ExpectActualMemberDiff.Kind.VarargChangedInOverride
-    ExpectActualCheckingCompatibility.Visibility -> ExpectActualMemberDiff.Kind.VisibilityChangedInOverride
-}