[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