compositor: remove unused signal

The move_signal in weston_output isn't used, and not even initialized,
so anything trying to listen to it will crash on wl_signal_add().
Instead of it, the 'output_moved_signal' in weston_compositor is
used, so remove it.

Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

src/compositor.h

@@ -200,7 +200,6 @@ struct weston_output {
 	int dirty;
 	struct wl_signal frame_signal;
 	struct wl_signal destroy_signal;
-	struct wl_signal move_signal;
 	int move_x, move_y;
 	uint32_t frame_time;
 	int disable_planes;