#433: duplicate group regsitrations were not being deleted when the user clicked "Well Never Mind Then"

This was also a bug last year that we just never noticed, but it's still worth fixing.  When deleting a not-yet-paid-for registration we pass the id of the thing we're deleting.  So for an attendee registration we pass the attendee id, and for a group registration we pass the group id.  But we didn't think to do that here, so it was only working for attendees and not groups.