Revert "object.c (mrb_eql): avoid mrb_funcall_argv() when possible"

This reverts commit bc012deef9.
Ref #6168

src/object.c

@@ -646,7 +646,6 @@ MRB_API mrb_bool
 mrb_eql(mrb_state *mrb, mrb_value obj1, mrb_value obj2)
 {
   if (mrb_obj_eq(mrb, obj1, obj2)) return TRUE;
-  if (mrb_class(mrb, obj1) != mrb_class(mrb, obj2)) return FALSE;
   return mrb_test(mrb_funcall_argv(mrb, obj1, MRB_SYM_Q(eql), 1, &obj2));
 }