Remove recentParticipants from ConpherenceThread

Summary: We no longer display this any more in the UI, so go ahead and remove the callsites and db column.

Test Plan: New Room, with and without participants.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D17683

src/applications/conpherence/controller/ConpherenceListController.php

@@ -159,7 +159,6 @@ final class ConpherenceListController extends ConpherenceController {
         ->setViewer($user)
         ->withPHIDs($conpherence_phids)
         ->needProfileImage(true)
-        ->needParticipantCache(true)
         ->execute();
 
       // this will re-sort by participation data