[kwineffects] Drop KWIN_GL_DEBUG

Summary:
It has been broken for many years and no one has filed bug
reports about it (besides me).

BUG: 419285

Test Plan: Ran kwin with KWIN_GL_DEBUG and it still works.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D28332