Drop QOffscreenSurface guard

Summary:
Given that our QPA now creates native offscreen surfaces, we no longer
need the QOffscreenSurface guard introduced by 61956025.

Reviewers: #kwin, romangg

Reviewed By: #kwin, romangg

Subscribers: romangg, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D22198