Use shorter variant of the same code

Signed-off-by: Mathias L. Baumann <[email protected]>

Author: Marenz

src/frequenz/dispatch/_actor_dispatcher.py

@@ -275,15 +275,10 @@ async def _stop_actor(self, stopping_dispatch: Dispatch, msg: str) -> None:
             stopping_dispatch: The dispatch that is stopping the actor.
             msg: The message to be passed to the actors being stopped.
         """
-        actor: Actor | None = None
         identity = self._dispatch_identity(stopping_dispatch)
 
-        actor = self._actors.get(identity)
-
-        if actor:
+        if actor := self._actors.pop(identity, None):
             await actor.stop(msg)
-
-            del self._actors[identity]
         else:
             _logger.warning(
                 "Actor for dispatch type %r is not running", stopping_dispatch.type