Remove unused usings

OpenRA.Game/Server/Server.cs

@@ -9,15 +9,12 @@
 #endregion
 
 using System;
-using System.Collections;
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
 using System.Net;
-using System.Net.NetworkInformation;
 using System.Net.Sockets;
 using System.Threading;
-using OpenRA.GameRules;
 using OpenRA.Graphics;
 using OpenRA.Network;
 using OpenRA.Primitives;