Don't grab the server when unmanaging the screen

There's no obvious reason for grabbing the X server when unmanaging
a screen - the only race conditions a server grab solves are those
related to querying and then acting on the results of the query.

Our shutdown sequence is correctly ordered according to the ICCCM -
we first unselect on the root window, and then we destroy the
window owning WM_S<n> so removing the grab should not cause any
problems when we are being replaced with another window manager.

https://bugzilla.gnome.org/show_bug.cgi?id=733068
MetaDisplay: remove meta_display_grab()/ungrab()

Now that we have two connections to the X server, the idea of a
ref-counted server grab that might be held across extended portions
of code is very dangerous since we might try to use the backend
connection while the frontend connection is grabbed.

Replace the only usage (which was local) with direct
XGrabServer/XUngrabServer usage and remove the meta_display_grab()
API.

https://bugzilla.gnome.org/show_bug.cgi?id=733068