Remove Twilio-PHP API external

Summary:
Ref T920. D19937 provides about 100 lines of code which can do essentially everything here; throw out the trillion lines of full external API stuff.

(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.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: aurelijus

Maniphest Tasks: T920

Differential Revision: https://secure.phabricator.com/D19938