res_xmpp: Fix ref counting issue

The only remaining reference to the endpoint is in the endpoints
container, and because it is unlinked in ast_endpoint_shutdown, we don't
have to explicitly cleanup the endpoint ourselves.

Change-Id: I912a2692e52d3e2ed445b32d8ae3f9004bc2f2e8

res/res_xmpp.c

@@ -561,7 +561,6 @@ static void xmpp_client_destructor(void *obj)
 	ast_xmpp_client_disconnect(client);
 
 	ast_endpoint_shutdown(client->endpoint);
-	ao2_cleanup(client->endpoint);
 	client->endpoint = NULL;
 
 	if (client->filter) {