Remove Compositor created check
Summary:
Cleans up the Compositor code some more. The check is only
used in a single assert statement.
Test Plan: Compiles.
Reviewers: KWin, zzag
Reviewed By: KWin, zzag
Subscribers: zzag, kwin
Tags: KWin
Maniphest Tasks: T11071
Differential Revision: https://phabricator.kde.org/D22278