Fix logic which should only apply for metaclasses in Module#undef_method

Author: eregon

src/main/java/org/truffleruby/core/module/ModuleFields.java

@@ -448,8 +448,7 @@ public void undefMethod(RubyContext context, Node currentNode, String methodName
         final InternalMethod method = ModuleOperations.lookupMethodUncached(rubyModuleObject, methodName, null);
         if (method == null || method.isUndefined()) {
             final DynamicObject moduleForError;
-
-            if (Layouts.CLASS.isClass(rubyModuleObject) && Layouts.CLASS.getIsSingleton(rubyModuleObject)) {
+            if (RubyGuards.isMetaClass(rubyModuleObject)) {
                 moduleForError = Layouts.CLASS.getAttached(rubyModuleObject);
             } else {
                 moduleForError = rubyModuleObject;