SQL patch to drop channel field.

Summary: Deleted the channel field and added a sql patch to drop the channel field.

Test Plan: I have messed up my local mysql:P, hence by storage upgrade is failing. Anyways, The chatlog_event table shouldn't contain the channel column now.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5011