R1:c5c2b8ce5a67

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…