Revert "Merge pull request #750 from drinkcat/xephyr-fullscreen"

Upstream bug has been fixed in xorg-xephyr 1.15.1, and this version is
now packaged in all affected releases.

This reverts commit f9ed015f64dacea14d70c019f62e00b88a81a803, reversing
changes made to 23bb77d4abb1b8e876566129ea717b2796de323e.