Remove last uses of JRequest in Joomla

Combined with #1495 this should remove all bar one instance (in the remember me plugin - https://github.com/joomla/joomla-cms/blob/master/plugins/system/remember/remember.php#L36) of JRequest from Joomla - which paves the way for it's removal in J4.0 :D

http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=31675