[#28243] Fix for issue #28243 - remove the remember me option from the offline.php page (3.x)

http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=28243
As suggested by Brian Teeman and Dennis Hermatski, the correct
solution is to remove the « remember me » option from the offline.php page, as admin can be always auto-logged and forget that site is actually offline.