Remove "mailKey" from "PhabricatorRepositoryCommit"

Summary: Ref T13197. Ref T13065. This continues the gradual purge of dedicated "mailKey" columns in favor of shared infrastructure.

Test Plan:

• Ran migration.
• Visually inspected database.
• Grepped for mailKey.
• Added some comments, saw the daemons generate corresponding mail.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13197, T13065

Differential Revision: https://secure.phabricator.com/D19670