Revert "screen: Set a black background for testing purposes"

This reverts commit ec8ed1dbb04ce8169c0a38ddf066d5565117c587.

1) It turns out to add a momentary flicker from the transition
between the login screen and user session
2) It actually isn't needed anymore since bug 733026

https://bugzilla.gnome.org/show_bug.cgi?id=740377