[gnome-shell] keyboard: Remove stale references to global.screen

[Carlos Garnacho <carlosg src gnome org>]

commit e3ebc8d0c6de7aa2d73e5aad9878a1c8dc17cb5f
Author: Carlos Garnacho <carlosg gnome org>
Date:   Wed Jul 11 18:46:49 2018 +0200

    keyboard: Remove stale references to global.screen
    
    (Re)introduced by the previous patch.

 js/ui/keyboard.js | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/js/ui/keyboard.js b/js/ui/keyboard.js
index 900372072..41942eeaa 100644
--- a/js/ui/keyboard.js
+++ b/js/ui/keyboard.js
@@ -492,8 +492,8 @@ var FocusTracker = new Lang.Class({
         this._currentWindow = null;
         this._currentWindowPositionId = 0;
 
-        global.screen.get_display().connect('notify::focus-window', () => {
-            this._setCurrentWindow(global.screen.get_display().focus_window);
+        global.display.connect('notify::focus-window', () => {
+            this._setCurrentWindow(global.display.focus_window);
             this.emit('window-changed', this._currentWindow);
         });