Update chat to work with new API version

osu.Game/Online/API/Requests/LeaveChannelRequest.cs

@@ -0,0 +1,31 @@
+// Copyright (c) 2007-2018 ppy Pty Ltd <contact@ppy.sh>.
+// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+using System.Net.Http;
+using osu.Framework.IO.Network;
+using osu.Game.Online.Chat;
+using osu.Game.Users;
+
+namespace osu.Game.Online.API.Requests
+{
+    public class LeaveChannelRequest : APIRequest
+    {
+        private readonly Channel channel;
+        private readonly User user;
+
+        public LeaveChannelRequest(Channel channel, User user)
+        {
+            this.channel = channel;
+            this.user = user;
+        }
+
+        protected override WebRequest CreateWebRequest()
+        {
+            var req = base.CreateWebRequest();
+            req.Method = HttpMethod.Delete;
+            return req;
+        }
+
+        protected override string Target => $@"chat/channels/{channel.Id}/users/{user.Id}";
+    }
+}