After _unmanage the object is semantically dead even if technically it's not,
so remove the prefs listener here to prevent it being called for a dead
object.

In particular this fixes a crash when starting up gnome-shell with at least
one gimp utility window opened which causes mutter to create a MetaWindow for
it only to immediately get an UnmapNotify afterwards which causes mutter to
unmanage the MetaWindow. Afterwards prefs_changed_callback is called for this
dead MetaWindow and tries to dereference the window->monitor pointer which is
already NULL.

https://bugzilla.gnome.org/show_bug.cgi?id=671087