res_pjsip_outbound_registration.c: Use ast_sorcery_object_unregister() API

The sorcery pjsip 'registration' config object needs to be destroyed on
module unload.  Otherwise, a reload of res_pjsip could try to use
callbacks for a previously unloaded instance of the module provided by
ast_sorcery_object_register() or one of the variants.  Also, if
res_pjsip_outbound_registration were subsequently reloaded, the sorcery
config field objects would be registered in sorcery twice.

ASTERISK-24907
Reported by: Kevin Harwell

Change-Id: I304fad13dece2604af48353f6c6d9d5c7b064697

res/res_pjsip_outbound_registration.c

@@ -1775,6 +1775,8 @@ static int unload_module(void)
 	ast_sorcery_observer_remove(ast_sip_get_sorcery(), "auth", &observer_callbacks_auth);
 	ast_sorcery_instance_observer_remove(ast_sip_get_sorcery(), &observer_callbacks_registrations);
 
+	ast_sorcery_object_unregister(ast_sip_get_sorcery(), "registration");
+
 	ao2_global_obj_release(current_states);
 
 	return 0;