Undo D6266 (DifferentialComment PHID migration)

Summary:
Ref T2222. My path forward here wasn't very good -- I was thinking I could set `transactionPHID` for the inline comments as I migrated, but it must be unique and an individual DifferentialComment may have more than one inline comment. Dropping the unique requirement just creates more issues for us, not fewer.

So the migration in D6266 isn't actually useful. Undo it -- this can't be a straight revert because some installs may already have upgraded.

Test Plan: Ran new migrations, verified the world ended up back in the same place as before (made comments, viewed reivsions).

Reviewers: btrahan

Reviewed By: btrahan

CC: wez, aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D6269