[IRFakeOverrides] Do not override modality in expect classes

It was a workaround for case, where expect class don't have
override of abstract function. We don't allow it anymore,
so it can be dropped to make correct modality comparation.

^KT-65971

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/overrides/IrFakeOverrideBuilder.kt

@@ -264,7 +264,6 @@ class IrFakeOverrideBuilder(
 
     private fun determineModalityForFakeOverride(
         members: List<FakeOverride>,
-        current: IrClass
     ): Modality {
         // Optimization: avoid creating hash sets in frequent cases when modality can be computed trivially
         var hasOpen = false
@@ -278,34 +277,25 @@ class IrFakeOverrideBuilder(
             }
         }
 
-        // Fake overrides of abstract members in non-abstract expected classes should not be abstract, because otherwise it would be
-        // impossible to inherit a non-expected class from that expected class in common code.
-        // We're making their modality that of the containing class, because this is the least confusing behavior for the users.
-        // However, it may cause problems if we reuse resolution results of common code when compiling platform code (see KT-15220)
-        val transformAbstractToClassModality =
-            current.isExpect && current.modality !== Modality.ABSTRACT && current.modality !== Modality.SEALED
         if (hasOpen && !hasAbstract) {
             return Modality.OPEN
         }
         if (!hasOpen && hasAbstract) {
-            return if (transformAbstractToClassModality) current.modality else Modality.ABSTRACT
+            return Modality.ABSTRACT
         }
 
         val realOverrides = members
             .map { it.original }
             .collectAndFilterRealOverrides()
-        return getMinimalModality(realOverrides, transformAbstractToClassModality, current.modality)
+        return getMinimalModality(realOverrides)
     }
 
     private fun getMinimalModality(
         members: Collection<IrOverridableMember>,
-        transformAbstractToClassModality: Boolean,
-        classModality: Modality
     ): Modality {
         var result = Modality.ABSTRACT
         for (member in members) {
-            val effectiveModality =
-                if (transformAbstractToClassModality && member.modality === Modality.ABSTRACT) classModality else member.modality
+            val effectiveModality = member.modality
             if (effectiveModality < result) {
                 result = effectiveModality
             }
@@ -334,7 +324,7 @@ class IrFakeOverrideBuilder(
         addedFakeOverrides: MutableList<IrOverridableMember>,
         compatibilityMode: Boolean
     ) {
-        val modality = determineModalityForFakeOverride(overridables, currentClass)
+        val modality = determineModalityForFakeOverride(overridables)
         val visibility = findMemberWithMaxVisibility(overridables).override.visibility
         val mostSpecific = selectMostSpecificMember(overridables)