datetime: fix field arrange code for dynamically field sequence change case.

Summary:
This is for support dynamically field sequence changing case

for example
User sets datetime format as "%d/%b/%y" first.
Then change the format dynamically in runtime to "%I:%M %p"

Previous format datetime items still there.
It breaks view(object dulicated on same geometry)

@fix

Test Plan:
Run datetime sample.
Add changed callback.
Change format in changed callback function.
Check the bug state.

Reviewers: jaehwan, id213sin, Hermet, cedric

Reviewed By: cedric

Differential Revision: https://phab.enlightenment.org/D3340

Signed-off-by: Cedric BAIL <cedric@osg.samsung.com>

legacy/elementary/src/lib/elm_datetime.c

@@ -231,7 +231,7 @@ _field_list_arrange(Evas_Object *obj)
 
         if (field->visible && field->fmt_exist)
           {
-             elm_layout_content_unset(obj, buf);
+             evas_object_hide(elm_layout_content_unset(obj, buf));
              elm_layout_content_set(obj, buf, field->item_obj);
           }
         else