Remove mailing lists application

Summary: Ref T8387. This is now completely obsoleted by mailing list users.

Test Plan: Grepped for mailinglist and related symbols.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: eadler, epriestley

Maniphest Tasks: T8387

Differential Revision: https://secure.phabricator.com/D13129