Do not attempt to stop watching user presence when requested to disconnect

First of all, this sort of cleanup isn't really the client's
responsibility, and secondly, at the point the client received this
request to disconnect, *none of its requests will be honored anymore*
(currently the only scenario of this if another client has connected
- the server-side concurrency filter will reject this request).

When disconnection is requested, the only valid thing to do with respect
to talking to the server is to stop doing it.

This will be moved server-side in a follow-up change, although I'm not
even strictly sure that's required - I'd like to think signalr would
know to clean up a disconnecting client from all groups they were in.

osu.Game/Online/Metadata/OnlineMetadataClient.cs

@@ -232,7 +232,6 @@ public override async Task EndWatchingUserPresence()
         public override async Task DisconnectRequested()
         {
             await base.DisconnectRequested().ConfigureAwait(false);
-            await EndWatchingUserPresence().ConfigureAwait(false);
             if (connector != null)
                 await connector.Disconnect().ConfigureAwait(false);
         }