R1:3124838d6567

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.