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.