Fix accidental gvc downgrade
ScreenShield: remove obsolete comment and hack

We don't need to wait to until the stage window is mapped to take
the modal grab, because that code now runs in a startup-prepared
signal handler, which in turn runs some time after the mainloop
has started and well after the stage window is mapped.

https://bugzilla.gnome.org/show_bug.cgi?id=711682