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

resources/sql/autopatches/20170413.conpherence.01.recentparty.sql

@@ -0,0 +1,2 @@
+ALTER TABLE {$NAMESPACE}_conpherence.conpherence_thread
+  DROP COLUMN recentParticipantPHIDs;