Remove all "Phabricator Bot" code

Summary:
Closes T7829 as wontfix. Closes T7965 as wontfix. Closes T7800 as wontfix. Closes T2731 as wontfix. Closes T1271 as wontfix.

We aren't maintaining this at all (see, e.g., T7829) and a user reported a technically accurate security issue via HackerOne: https://hackerone.com/reports/222870

Just throw it away until we get to the eventual Conphernece bot/API update and can do this stuff correctly.

Test Plan: Grepped for phabricatorbot.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7965, T7829, T7800, T2731, T1271

Differential Revision: https://secure.phabricator.com/D17756