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