res_pjsip_outbound_registration: Clean up state when registration is deleted

res_pjsip_outbound_registration:  Clean up state when registration is deleted

Nothing was cleaning up the registration state object when ast_sorcery_delete
was called on a registration.  So, the registration was deleted from sorcery
but the state object went right on refreshing the registration (or failing
to refresh the registration) with the peer.

* Added a 'deleted' observer on registration that removes the state object.

ASTERISK-25964 #close
Reported-by Matt Jordan

Change-Id: I2db792145cdb1f72ebbf57dd9099596dbbf12c23