Revert showing notifications in the locked screen

As PAM messages are now shown below the password entry, there is no
need for this complexity, and we can just hide all notifications.
Also, this avoids the ambiguity between notification.showWhenLocked and
source.showInLockScreen, which have very different effects.

https://bugzilla.gnome.org/show_bug.cgi?id=683369

js/ui/overview.js

@@ -71,7 +71,6 @@ const ShellInfo = new Lang.Class({
         if (this._source.notifications.length == 0) {
             notification = new MessageTray.Notification(this._source, text, null);
             notification.setTransient(true);
-            notification.setShowWhenLocked(true);
         } else {
             notification = this._source.notifications[0];
             notification.update(text, null, { clear: true });