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