screenShield: Remove lock screen group from Crtl-Alt-Tab manager

There is nothing else to be focused in the lock screen itself -- the
top bar is already handled elsewhere, and the dialog manages itself
now.

Remove the lock screen group from the Ctrl-Alt-Tab manager.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/872

js/ui/screenShield.js

@@ -55,7 +55,6 @@ var ScreenShield = class {
             name: 'lockScreenGroup',
             visible: false,
         });
-        Main.ctrlAltTabManager.addGroup(this._lockScreenGroup, _("Lock"), 'changes-prevent-symbolic');
 
         this._lockDialogGroup = new St.Widget({
             x_expand: true,