Don't write inverse edges for DifferentialRevisionHasReviewerEdgeType

Summary: The default behavior was inadvertedly changed in D11074. This restores the original behavior.

Test Plan: Added a project reviewer to a diff, saw no inverse transaction recorded.

Reviewers: Krenair, epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11181