[FIR] Remove SessionUtils because it's not used

compiler/fir/semantics/src/org/jetbrains/kotlin/fir/SessionUtils.kt

@@ -1,38 +0,0 @@
-/*
- * Copyright 2010-2021 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.fir
-
-import org.jetbrains.kotlin.descriptors.Visibility
-import org.jetbrains.kotlin.fir.declarations.FirClass
-import org.jetbrains.kotlin.fir.declarations.FirSimpleFunction
-import org.jetbrains.kotlin.fir.declarations.utils.visibility
-import org.jetbrains.kotlin.fir.resolve.ScopeSession
-import org.jetbrains.kotlin.fir.scopes.ProcessorAction
-import org.jetbrains.kotlin.fir.scopes.processOverriddenFunctions
-import org.jetbrains.kotlin.fir.scopes.unsubstitutedScope
-
-/**
- * Returns the list of functions that overridden by given
- */
-fun FirSimpleFunction.lowestVisibilityAmongOverrides(
-    containingClass: FirClass,
-    session: FirSession,
-    scopeSession: ScopeSession
-): Visibility {
-    val firTypeScope = containingClass.unsubstitutedScope(session, scopeSession, withForcedTypeCalculator = false)
-    var visibility = visibility
-
-    // required; otherwise processOverriddenFunctions()
-    // will process nothing
-    firTypeScope.processFunctionsByName(symbol.fir.name) { }
-
-    firTypeScope.processOverriddenFunctions(symbol) {
-        visibility = it.fir.visibility
-        ProcessorAction.NEXT
-    }
-
-    return visibility
-}