Remove standalone SMS support in favor of a "Mail, SMS, and other media are mostly the same thing" approach

Summary:
Ref T920. Over time, mail has become much more complex and I think considering "mail", "sms", "postcards", "whatsapp", etc., to be mostly-the-same is now a more promising avenue than building separate stacks for each one.

Throw away all the standalone SMS code, including the Twilio config options. I have a separate diff that adds Twilio as a mail adapter and functions correctly, but it needs some more work to bring upstream.

This permanently destroys the sms table, which no real reachable code ever wrote to. I'll call this out in the changelog.

Test Plan:

• Grepped for SMS and Twilio.
• Ran storage upgrade.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T920

Differential Revision: https://secure.phabricator.com/D19939