evas: Move clip_unset to legacy, remove group_clip

Similarly to group_color_set, group_clip_[un]set should not
exist and should be a result of efl_super and inheritance.

This patch also removes clip_unset from the EO API and keeps
only clip_set(NULL). The reason is that it will avoid bad overrides
of clip_unset() vs. clip_unset(NULL). This also simplifies the code
a bit. Ideally we should be able to reintroduce clip_unset in EO
if we can have a "@final" tag (like java's final keyword), to
prevent overrides.

unsorted/evas/evas-object-manipulation-eo.c

@@ -129,7 +129,7 @@ _on_keydown(void        *data EINA_UNUSED,
         clip = efl_canvas_object_clip_get(d.img);
         if (clip == d.clipper)
           {
-             efl_canvas_object_clip_unset(d.img);
+             efl_canvas_object_clip_set(d.img, NULL);
              fprintf(stdout, "off\n");
           }
         else