Remove incorrect removal of iv

src/class.c

@@ -1417,13 +1417,10 @@ mrb_mod_alias(mrb_state *mrb, mrb_value mod)
 static void
 undef_method(mrb_state *mrb, struct RClass *c, mrb_sym a)
 {
-  mrb_value m, val;
+  mrb_value m;
 
   if (!mrb_obj_respond_to(c, a)) {
-    val = mrb_iv_remove(mrb, mrb_obj_value(c), a);
-    if (mrb_undef_p(val)) {
-      mrb_name_error(mrb, a, "undefined method '%S' for class '%S'", mrb_sym2str(mrb, a), mrb_obj_value(c));
-    }
+    mrb_name_error(mrb, a, "undefined method '%S' for class '%S'", mrb_sym2str(mrb, a), mrb_obj_value(c));
   } else {
     MRB_SET_VALUE(m, MRB_TT_PROC, value.p, 0);
     mrb_define_method_vm(mrb, c, a, m);