efl.ui.widget: remove widget_top property

Summary:
this can be mimicked with efl_provider_find as the top widget should
always be efl.ui.win

ref T7553
Depends on D8063

Reviewers: cedric

Reviewed By: cedric

Subscribers: cedric, reviewers, committers

Tags: efl: api

Maniphest Tasks: T7553

Differential Revision: https://phab.enlightenment.org/D8064