compositor: Eliminate marshalling warning for leave events

compositor: Eliminate marshalling warning for leave events

Don't NULL the resource pointer before calling weston_surface_destroy().
We use to have more of a distinction between compositor created surfaces
and client surfaces, and weston_surface_destroy couldn't be used for
client surfaces.  Now it all goes through weston_surface_destroy() and
we can remove the assert and the NULL-ing of resource, which caused the
marshalling warning.