Remove old code for "Request Review" action from Differential

Summary: Ref T10967. This moves all remaining "request review" pathways (just differential.createcomment) to the new code, and removes the old action.

Test Plan: Requested review on a revision, grep'd for the action constant.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10967

Differential Revision: https://secure.phabricator.com/D17567