Remove __after_morecore_hook

Remove __after_morecore_hook from the API and finalize the symbol so
that it can no longer be used in new applications.  Old applications
using __after_morecore_hook will find that their hook is no longer
called.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>