add !mp removeref and !mp listrefs

[ILW8]

I didn't know these existed on bancho

[Copilot]

Pull Request Overview

This PR adds new multiplayer referee management commands (!mp removeref and !mp listrefs) to complement the existing !mp addref functionality in the osu! client.

• Extends referee management with removal and listing capabilities
• Refactors existing addref logic to use a switch statement for better command organization
• Updates method signatures to return success/failure status for better command feedback

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
MultiplayerMatchSubScreen.cs	Modifies AddRef to return boolean success status and adds RemoveRef methods with overloads for username and APIUser
StandAloneChatDisplay.cs	Adds removeref and listrefs commands, refactors command handling to use switch statement, and improves logging

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