Remove Twilio-PHP API external
(I am generally not sure why everyone writes API libraries like this instead of like D19937.)
Test Plan: Send SMS messages with D19937, so I don't think we need any of this code anymore. This code is techncially reachable through some pathways like bin/sms, but won't be for long.
Reviewed By: amckinley
Maniphest Tasks: T920
Differential Revision: https://secure.phabricator.com/D19938