[FIR] Get rid of FirSerializerExtension.customClassMembersProducer

This service was needed in FE 1.0 because of different scope structure
  for Java classes, but seems like it has no sense for FIR

compiler/fir/fir-serialization/src/org/jetbrains/kotlin/fir/serialization/FirElementSerializer.kt

@@ -170,10 +170,8 @@ class FirElementSerializer private constructor(
             }
         }
 
-        val callableMembers =
-            extension.customClassMembersProducer?.getCallableMembers(klass)
-                ?: (klass.memberDeclarations() + providedDeclarationsService.getProvidedCallables(classSymbol, scopeSession))
-                    .sortedWith(FirCallableDeclarationComparator)
+        val callableMembers = (klass.memberDeclarations() + providedDeclarationsService.getProvidedCallables(classSymbol, scopeSession))
+            .sortedWith(FirCallableDeclarationComparator)
 
         for (declaration in callableMembers) {
             if (declaration !is FirEnumEntry && declaration.isStatic) continue // ??? Miss values() & valueOf()

compiler/fir/fir-serialization/src/org/jetbrains/kotlin/fir/serialization/FirSerializerExtension.kt

@@ -99,11 +99,4 @@ abstract class FirSerializerExtension {
     open fun serializeErrorType(type: ConeErrorType, builder: ProtoBuf.Type.Builder) {
         throw IllegalStateException("Cannot serialize error type: $type")
     }
-
-    open val customClassMembersProducer: ClassMembersProducer?
-        get() = null
-
-    interface ClassMembersProducer {
-        fun getCallableMembers(klass: FirClass): Collection<FirCallableDeclaration>
-    }
 }

compiler/fir/fir-serialization/src/org/jetbrains/kotlin/fir/serialization/FirSerializerExtensionBase.kt

@@ -100,9 +100,6 @@ abstract class FirSerializerExtensionBase(
         typeParameter.serializeAnnotations(proto, protocol.typeParameterAnnotation)
     }
 
-    override val customClassMembersProducer: ClassMembersProducer?
-        get() = super.customClassMembersProducer
-
     @Suppress("Reformat")
     private fun <
         MessageType : GeneratedMessageLite.ExtendableMessage<MessageType>,