IR: remove IdSignatureBuilder.isKotlinPackage()

compiler/ir/serialization.common/src/org/jetbrains/kotlin/backend/common/serialization/signature/IdSignatureBuilder.kt

@@ -106,8 +106,6 @@ abstract class IdSignatureBuilder<D> {
     protected open fun platformSpecificAlias(descriptor: TypeAliasDescriptor) {}
     protected open fun platformSpecificPackage(descriptor: PackageFragmentDescriptor) {}
 
-    protected open fun isKotlinPackage(descriptor: PackageFragmentDescriptor): Boolean = true
-
     fun buildSignature(declaration: D): IdSignature {
         reset()
 

compiler/ir/serialization.common/src/org/jetbrains/kotlin/backend/common/serialization/signature/IdSignatureDescriptor.kt

@@ -54,7 +54,7 @@ open class IdSignatureDescriptor(private val mangler: KotlinMangler.DescriptorMa
             get() = visibility == DescriptorVisibilities.PRIVATE
 
         private val DeclarationDescriptorWithVisibility.isTopLevelPrivate: Boolean
-            get() = isPrivate && mangler.run { !isPlatformSpecificExport() } && containingDeclaration?.let { it is PackageFragmentDescriptor && isKotlinPackage(it) } ?: false
+            get() = isPrivate && mangler.run { !isPlatformSpecificExport() } && containingDeclaration is PackageFragmentDescriptor
 
         override fun visitPackageFragmentDescriptor(descriptor: PackageFragmentDescriptor, data: Nothing?) {
             packageFqn = descriptor.fqName