Remove unused parameter from PhabricatorMailEmailEngine::newEmailThreadingHeaders() call

Summary:
Method PhabricatorMailEmailEngine::newEmailThreadingHeaders() invoked with 1 parameter, 0 required.
Thus remove the parameter from its call in the same class.
The called private function does $mailer = $this->getMailer() anyway.

Test Plan: Read 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/D25747