Remove half-baked thread-safety mechanisms from Sheet.

Sheet is now thread-unsafe, rather than attempting to expose a dodgy thread safety model.

OpenRA.Game/Graphics/SpriteFont.cs

@@ -136,7 +136,7 @@ namespace OpenRA.Graphics
 					}
 				}
 
-			s.Sheet.CommitData();
+			s.Sheet.CommitBufferedData();
 
 			return g;
 		}