Remove X11Compositor requires compositing check

Summary:
With the split of the Compositor class in X11 and Wayland it is unnecessary
to check in X11Compositor if compositing is required since on X it never is.

Test Plan: Compiles.

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D23032