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