keyboard: don't raise to the top when showing

The stacking is already correct due to the grouping, and calling
raise_top() results in the keyboard above the OSDs.

https://bugzilla.gnome.org/show_bug.cgi?id=700620

js/ui/layout.js

@@ -686,7 +686,6 @@ const LayoutManager = new Lang.Class({
     },
 
     showKeyboard: function () {
-        this.keyboardBox.raise_top();
         Tweener.addTween(this.keyboardBox,
                          { anchor_y: this.keyboardBox.height,
                            time: KEYBOARD_ANIMATION_TIME,