Remove composite reset timer

Summary:
To streamline Compositor code more remove the composite reset timer. The two
times it was used we can either use a singleshot timer instead or connect the
call to a different signal in the X11 backend.

Long term goal is to have a well structured init of the Compositor such that
we can call directly instead.

Test Plan: Manually in X and Wayland nested session.

Reviewers: KWin, zzag

Reviewed By: KWin, zzag

Subscribers: zzag, kwin

Tags: KWin

Maniphest Tasks: T11071

Differential Revision: https://phabricator.kde.org/D22270