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