Remove incorrect policy language on Diff reviewers

Summary: Fixes T12952. This never work AFAIK, so resolves this mis-information. See T4411 for follow up.

Test Plan: Click on policy for a diff, no longer see text.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12952

Differential Revision: https://secure.phabricator.com/D18349