Fix discord RPC errors in multiplayer

Reproduction steps:

1. Go to multiplayer
2. Create a room
3. Play a map to completion
4. Wait for "secrets cannot currently be sent with buttons" error
   messages

The fix is to clear the buttons since they're the less important ones.

osu.Desktop/DiscordRichPresence.cs

@@ -191,6 +191,9 @@ private void updatePresence(bool hideIdentifiableInformation)
                 };
 
                 presence.Secrets.JoinSecret = JsonConvert.SerializeObject(roomSecret);
+                // discord cannot handle both secrets and buttons at the same time, so we need to choose something.
+                // the multiplayer room seems more important.
+                presence.Buttons = null;
             }
             else
             {