Delete URL-Parameter limitstart (also when its value is 0)

osmetic change for consistence and avoidance of duplicate URLs.
 See also this Bug report: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=25616&start=0
 Before change $limitstart was deleted if present, except if it was 0 (zero), because 0 evaluates to false.