Remove custom Stopwatch wrapper

Remove the redirection (that doesn't offer any new functionality)
and replace it with the familiar System.Diagnostics.Stopwatch.

OpenRA.Game/Support/PerfHistory.cs

@@ -10,6 +10,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Drawing;
 using OpenRA.Primitives;
 
@@ -106,7 +107,7 @@ namespace OpenRA.Support
 
 	public class PerfSample : IDisposable
 	{
-		readonly Stopwatch sw = new Stopwatch();
+		readonly Stopwatch sw = Stopwatch.StartNew();
 		readonly string Item;
 
 		public PerfSample(string item)

OpenRA.Game/Support/PerfTimer.cs

@@ -9,6 +9,7 @@
 #endregion
 
 using System;
+using System.Diagnostics;
 using System.Linq;
 using System.Threading;
 
@@ -16,7 +17,7 @@ namespace OpenRA.Support
 {
 	public class PerfTimer : IDisposable
 	{
-		readonly Stopwatch sw = new Stopwatch();
+		readonly Stopwatch sw = Stopwatch.StartNew();
 		readonly string Name;
 
 		//

OpenRA.Game/Support/Stopwatch.cs

@@ -1,32 +0,0 @@
-#region Copyright & License Information
-/*
- * Copyright 2007-2014 The OpenRA Developers (see AUTHORS)
- * This file is part of OpenRA, which is free software. It is made
- * available to you under the terms of the GNU General Public License
- * as published by the Free Software Foundation. For more information,
- * see COPYING.
- */
-#endregion
-
-namespace OpenRA.Support
-{
-	public class Stopwatch
-	{
-		System.Diagnostics.Stopwatch sw;
-
-		public Stopwatch()
-		{
-			Reset();
-		}
-
-		public System.TimeSpan Elapsed
-		{
-			get { return this.sw.Elapsed; }
-		}
-
-		public void Reset()
-		{
-			sw = System.Diagnostics.Stopwatch.StartNew();
-		}
-	}
-}