R1:5587abf04c8b

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