wayland: Replace buffer destroy wl_signal with a GObject signal

Don't use the libwayland-* utilities when we have our own that do the
same thing.

https://bugzilla.gnome.org/show_bug.cgi?id=762828

src/wayland/meta-wayland-buffer.h

@@ -36,7 +36,6 @@ struct _MetaWaylandBuffer
   GObject parent;
 
   struct wl_resource *resource;
-  struct wl_signal destroy_signal;
   struct wl_listener destroy_listener;
 
   CoglTexture *texture;