Remove compositing initialized check

Summary:
The check is used to limit settings reload. But we can afford to reload
settings in case the compositing gets toggled since this happens rarely.

Removing the check reduces code complexity.

Test Plan: Manually in X and Wayland.

Reviewers: KWin, zzag

Reviewed By: KWin, zzag

Subscribers: kwin

Tags: KWin

Maniphest Tasks: T11071

Differential Revision: https://phabricator.kde.org/D22220