efl_ui_layout: call efl_del instead of efl_unref when text part does not exist

Summary:
Because `efl_isa` doesn't call _efl_object_call_end, the part object is not
removed. it occurs a bunch of efl_unref error message.

Test Plan:
1. elementary_test -to 'efl.ui.box'
2. exit window.
3. check there is no efl_unref error message.

Reviewers: zmike, cedric

Subscribers: #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D8385

src/lib/elementary/efl_ui_layout.c

@@ -2770,7 +2770,7 @@ elm_layout_text_set(Eo *obj, const char *part, const char *text)
    if (!efl_isa(part_obj, EFL_TEXT_INTERFACE) ||
        !efl_isa(part_obj, EFL_UI_LAYOUT_PART_CLASS))
      {
-        efl_unref(part_obj);
+        efl_del(part_obj);
         return EINA_FALSE;
      }