compositor: remove the authenticate vfunc

It was introduced in 5fcd0aa58e and
then used in code that doesn't exist anymore.

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

src/compositor.h

@@ -673,7 +673,6 @@ struct weston_compositor {
 
 	void (*destroy)(struct weston_compositor *ec);
 	void (*restore)(struct weston_compositor *ec);
-	int (*authenticate)(struct weston_compositor *c, uint32_t id);
 
 	struct weston_launcher *launcher;