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
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