Revert "grab only the registrants from given session"

This reverts commit 137b73c7c0a469d3bf29da4fcc3e1721ecad88ca.
grab only the registrants from given session

the page /classes/staff/ has links to staff and registrations for a
session.  the previous incarnation is returning a list of *all*
registrations ever.  which is not only not helpful, but also timesout on
our server