efl_ui_widget: child_can_focus is not needed anymore

efl_ui_widget: child_can_focus is not needed anymore

summary_:
pd->logical.child_count is a counter where focusable children do
register themself in the parent if they can be focused or not. With this
we don't need the updating of this internal flag nor the flag itself anymore.

Depends on D8027

Reviewers: woohyun, cedric, Jaehyun_Cho, zmike, segfaultxavi

Reviewed By: segfaultxavi

Subscribers: #reviewers, #committers

Tags: #efl

Reviewed-by: Xavi Artigas <xavierartigas@yahoo.es>
Differential Revision: https://phab.enlightenment.org/D8030