[#34000] Remove re-definition of table key name

The table key name has already been assigned to `$key` in line 1064 and is good to be used again. There's no need to introduce another variable for the sake of model state update.

Associated [Bug tracker item](http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=34000)