[platforms/x11] Delete duplicate doxygen comments in X11Output

Summary:
geometry(), refreshRate(), and gammaRampSize() are already documented in
AbstractOutput class.

Test Plan: Compiles.

Reviewers: KWin, apol

Reviewed By: apol

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D23481