Bug 694570 - (4/4) - Drop unused Xatom.h include. r=karlt

Introduced by 12f125711253a30972cd2618ad9d8041a1796e0c for N900 support, which
was removed by 9d925d047ba50e5312ccab8b48b6b5237bb4e35b

Author: oliverhenshaw

toolkit/xre/nsNativeAppSupportUnix.cpp

@@ -34,7 +34,6 @@
 
 #ifdef MOZ_X11
 #include <gdk/gdkx.h>
-#include <X11/Xatom.h>
 #include <X11/ICE/ICElib.h>
 #include <X11/SM/SMlib.h>
 #include <fcntl.h>