Remove obsolete comment

src/lib/barrier/mouse_types.h

@@ -2,11 +2,11 @@
  * barrier -- mouse and keyboard sharing utility
  * Copyright (C) 2012-2016 Symless Ltd.
  * Copyright (C) 2002 Chris Schoeneman
- * 
+ *
  * This package is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License
  * found in the file LICENSE that should have accompanied this file.
- * 
+ *
  * This package is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
@@ -41,5 +41,4 @@ static const ButtonID   kMacButtonRight = 2;
 static const ButtonID   kMacButtonMiddle = 3;
 //@}
 
-// XXX -- some osx stuff uses this and might expect it to be 5
 static const UInt8      NumButtonIDs  = 6;