Partly remove Compositor restart functionality

Summary:
This removes the restart function of the Compositor class and renames the
internal reinitialize function.

Instead of the restart function reinitialize can be called. Reading again
the settings in this case is fine, since it is done rarely. This reduces
the code complexity.

Test Plan: Manually on Wayland and X. 100% autotests pass.

Reviewers: KWin, zzag

Reviewed By: KWin, zzag

Subscribers: davidedmundson, zzag, kwin

Tags: KWin

Maniphest Tasks: T11071

Differential Revision: https://phabricator.kde.org/D22225