Unregistering the display change listener looks like a pointless
excercise given we'll exit in a moment.  When exiting qemu via
menu/file/quit this will not happen either.  Just drop the code.

Also return TRUE unconditionally.  This will tell gtk to ignore the
close request, so gtk will not start destroying widgets and causing
warnings due to UI code trying to talk to widgets which are gone.
Just depend on qmp_quit() doing it's job instead.

Reported-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-Id: <20180314080439.4229-1-kraxel@redhat.com>