Drop Maniphest legacy transaction table
Summary: Ref T2217. I'll hold this for a month or so, but once we're confident the migration didn't ruin anything we should nuke this old data -- it's just an insurance policy against discovering migration issues.
Test Plan: Will run in a month or so.
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley, aran
Maniphest Tasks: T2217
Differential Revision: https://secure.phabricator.com/D7104