x11: Remove XComposite

x11: Remove XComposite

It's only used during DND to allow use of the root window's cow window
as a DND target, because apparently gnome-shell used to think that was a
great idea to DND to the overview.

Somebody complain to gnome-shell devs about it not being a good idea if
they want it fixed.
Potentially using Wayland is a better idea though.

This reverts 85ae875dcbfcfa3fbdca857dc90467d39b9ff0c5

Related: https://bugzilla.gnome.org/show_bug.cgi?id=601731