[Later] Drop legacyCommentID column from DifferentialTransactionComment
Summary: Ref T2222. No callsites. Holding until we're more clearly stable.
Test Plan: grep
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley, aran
Maniphest Tasks: T2222
Differential Revision: https://secure.phabricator.com/D8241