compositor: Don't remove surface->link in weston_surface_unmap()

Expecting the link to be initialized breaks when compositor-drm.c
takes surfaces out of the list.  We could fix that to also
wl_list_init() the link, but we're moving to making the surface list
local to weston_output_repaint(), rebuilt in and only valid during
weston_output_repaint() so just don't touch it instead.

src/compositor.c

@@ -658,14 +658,8 @@ weston_surface_unmap(struct weston_surface *surface)
 
 	weston_surface_damage_below(surface);
 	surface->output = NULL;
-	wl_list_remove(&surface->link);
 	wl_list_remove(&surface->layer_link);
 
-	/* If a surface is mapped and unmapped before a repaint occurs, it
-	 * won't be added to the compositor's surface list, so make sure the
-	 * call to wl_list_remove(&surface->link) won't fail next time */
-	wl_list_init(&surface->link);
-
 	wl_list_for_each(seat, &surface->compositor->seat_list, link) {
 		if (seat->seat.keyboard &&
 		    seat->seat.keyboard->focus == &surface->surface)