R1:e856e791f3ee

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

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