Remove "re prefix" and "vary subjects" config

Summary: Ref T11098. There is no reason to maintain these as separate values now that they can be configured in global settings.

Test Plan:

• Hit and read setup issue.
• Fiddled with settings.
• I'll vet this more throughly in the next diff since I need to fix an issue with global defaults in mail and can explicitly test this at the same time.

Reviewers: chad

Reviewed By: chad

Subscribers: eadler

Maniphest Tasks: T11098

Differential Revision: https://secure.phabricator.com/D16117