R1:a8af0c6e5119

Remove unused variable from HeraldEngine

Summary: `$activeRule` is not used anywhere.

Test Plan: Grep the code.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25679