fix: ensure liveupdate pubsubs exit (#5557)

Previously, the derived class (i.e. BttvLiveUpdates or SeventvEventAPI)
    would have their destructor ran before BasicPubSubManager called
    stop, meaning there was a time wherein messages could still flow
    through, attempting to call `onMessage` on a pure virtual, causing a
    crash.

src/providers/twitch/PubSubManager.cpp

@@ -581,8 +581,6 @@ void PubSub::stop()
             this->websocketClient.stop();
         }
     }
-
-    assert(this->clients.empty());
 }
 
 bool PubSub::listenToWhispers()