elm_popup: remove on_show_region_set call

Summary:
Its hard to say why removing such calls are fixing a bug like T7274.
However, it appears that this call causes the min size calculation of
evas to fall apart. This also removes a workarround introduced earlier
to fix this.

This fixes T7274

Reviewers: zmike

Reviewed By: zmike

Subscribers: cedric, reviewers, committers

Tags: efl

Maniphest Tasks: T7274

Differential Revision: https://phab.enlightenment.org/D8043