fix a use after free

This line makes dwl crash after closing mpv with the switchtotag patch.

dwl.c

@@ -1171,7 +1171,6 @@ void
 destroydecoration(struct wl_listener *listener, void *data)
 {
 	Client *c = wl_container_of(listener, c, destroy_decoration);
-	c->decoration = NULL;
 
 	wl_list_remove(&c->destroy_decoration.link);
 	wl_list_remove(&c->set_decoration_mode.link);