R1:c36b82c7b8a9

Remove unused PhabricatorMail class

Summary: This class is no longer used after D13032.

Test Plan: `grep`

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D13118