Remove unnecessary exception type

osu.Game/Online/RealtimeMultiplayer/AlreadyInRoomException.cs

@@ -1,23 +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;
-using System.Runtime.Serialization;
-using Microsoft.AspNetCore.SignalR;
-
-namespace osu.Game.Online.RealtimeMultiplayer
-{
-    [Serializable]
-    public class AlreadyInRoomException : HubException
-    {
-        public AlreadyInRoomException()
-            : base("This user is already in a multiplayer room.")
-        {
-        }
-
-        protected AlreadyInRoomException(SerializationInfo info, StreamingContext context)
-            : base(info, context)
-        {
-        }
-    }
-}