R1:9d5b933ed5a3

Remove all legacy configuration options for mailers

Summary:
Ref T920. About a year ago (in 2018 Week 6, see D19003) we moved from individually configured mailers to `cluster.mailers`, primarily to support fallback across multiple mail providers.

Since this has been stable for quite a while, drop support for the older options.

Test Plan: Grepped for all removed options.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T920

Differential Revision: https://secure.phabricator.com/D19940