Remove metamta.send-immediately

Summary:
Ref T3857.

• Always send mail via daemons. This lets us get rid of this config, and is generally much more performant.
• After D7964, we warn if daemons aren't running.

Test Plan: Sent some mail.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3857

Differential Revision: https://secure.phabricator.com/D7965