wayland: Fix output destroyed callback vfunc type

It is not a callback on a parameter signal, and get no GParamSpec passed
to it. This fixes a crash when a surface is on a destroyed output.

https://bugzilla.gnome.org/show_bug.cgi?id=755096

src/wayland/meta-wayland-surface.c

@@ -936,7 +936,6 @@ set_surface_is_on_output (MetaWaylandSurface *surface,
 
 static void
 surface_handle_output_destroy (MetaWaylandOutput *wayland_output,
-                               GParamSpec *pspec,
                                MetaWaylandSurface *surface)
 {
   set_surface_is_on_output (surface, wayland_output, FALSE);