Remove all application-specific reply handler class overrides

Summary:
Ref T7199. In the vein of D12231, these options were a bad idea.

• They once served a very narrow, Facebook-specific need (see T1992), except even Facebook only used the Differential setting AFAIK.
• Outside of that special case, they are unused and essentially unusable (generally speaking, they do not meaningfully implement anything modular or replaceable).
• I have no knowledge of any install ever changing these settings, and can imagine no reason why they would.

Moving forward:

• If they really need to, they can fork locally and chagne one line.
• I expect "!actions" to make mail at least somewhat more modular soon, anyway.
• Any derived handlers would break after T7199 and need to be rewritten anyway, so this is just taking advantage of a BC break to do cleanup.

Test Plan:

• Grepped for removed configuration.
• Sent some mail from applications, verified the reply handlers set proper reply addresses.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7199

Differential Revision: https://secure.phabricator.com/D12233