GDM currently saves a screenshot to a pixmap and puts its
XID in the _XROOTPMAP_ID root window property.

It does this because gnome-settings-daemon used to use the
_XROOTPMAP_ID to figure out a start frame to crossfade from.

g-s-d doesn't do that anymore, all background handling is
now down by gnome-shell, and gnome-shell does its own
screenshotting.

This commit strips that functionality from gdm.

https://bugzilla.gnome.org/show_bug.cgi?id=694291