Remove stale #include <X11/Xlib.h> to unbreak Freon builds

BUG=488739
TBR=derat@chromium.org

Review URL: https://codereview.chromium.org/1137393004

Cr-Commit-Position: refs/heads/master@{#330270}

ash/accelerators/key_hold_detector.cc

@@ -4,11 +4,6 @@
 
 #include "ash/accelerators/key_hold_detector.h"
 
-#include <X11/Xlib.h>
-
-#undef RootWindow
-#undef Status
-
 #include "ash/shell.h"
 #include "base/message_loop/message_loop.h"
 #include "ui/aura/window_tracker.h"