Kotlin: Small simplification

Also removes a cast

Author: igfoo

java/kotlin-extractor/src/main/kotlin/KotlinUsesExtractor.kt

@@ -1416,12 +1416,11 @@ open class KotlinUsesExtractor(
         for(t in subbedSupertypes) {
             when(t) {
                 is IrSimpleType -> {
-                    when (t.classifier.owner) {
+                    val owner = t.classifier.owner
+                    when (owner) {
                         is IrClass -> {
-                            val classifier: IrClassifierSymbol = t.classifier
-                            val tcls: IrClass = classifier.owner as IrClass
                             val typeArgs = if (t.arguments.isNotEmpty() && mode is ExtractSupertypesMode.Raw) null else t.arguments
-                            val l = useClassInstance(tcls, typeArgs, inReceiverContext).typeResult.id
+                            val l = useClassInstance(owner, typeArgs, inReceiverContext).typeResult.id
                             tw.writeExtendsReftype(id, l)
                         }
                         else -> {