R1:77601bf58c9a

Remove "Reviewed By" Differential field

Summary: Ref T11114. This is replaced by `DifferentialReviewedByCommitMessageField.php`.

Test Plan:
- Used `differential.getcommitmessage` to query an accepted revision, saw "Reviewed By".
- Grepped for removed class name.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17079