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