Always call `Actor.cancel()` at end of root's main task

It's simpler and the only real logical difference is logging messages.
This should also give us an overall consistent tear down sequence.

tractor/_actor.py

@@ -1083,20 +1083,13 @@ async def _start_actor(
         try:
             result = await main()
         except (Exception, trio.MultiError) as err:
-            try:
             log.exception("Actor crashed:")
             await _debug._maybe_enter_pm(err)
 
             raise
-
         finally:
             await actor.cancel()
 
-        # XXX: the actor is cancelled when this context is complete
-        # given that there are no more active peer channels connected
-        actor.cancel_server()
-        actor._service_n.cancel_scope.cancel()
-
     # unset module state
     _state._current_actor = None
     log.info("Completed async main")