Remove all application-specific reply handler domains

Summary:
Ref T7199. These were a bad idea which got copy-pasted a bunch.

• There is zero reason to ever set these to different things.
• Unsurprisingly, I don't know of any install which has them set to different things.

Unless I've completely forgotten about it, this option was not motivated by some obscure business need, it was just a bad decision which didn't catch anyone's attention at the time.

We partially remedied the mistake at some point by introducing metamta.reply-handler-domain, which works as a default for all applications, but never cleaned this mess up.

Test Plan: Sent some mail from applications, verified it picked up appropraite reply handler domains.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7199

Differential Revision: https://secure.phabricator.com/D12231