Move client state class to server implementation

Never used by client.

osu.Game/Online/RealtimeMultiplayer/MultiplayerClientState.cs

@@ -1,20 +0,0 @@
-// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
-// See the LICENCE file in the repository root for full licence text.
-
-using System;
-
-#nullable enable
-
-namespace osu.Game.Online.RealtimeMultiplayer
-{
-    [Serializable]
-    public class MultiplayerClientState
-    {
-        public readonly long CurrentRoomID;
-
-        public MultiplayerClientState(in long roomId)
-        {
-            CurrentRoomID = roomId;
-        }
-    }
-}