Remove support for GitHub post-receive notifications

Summary:

• These never actually did anything.
• I don't even really remember why I built them, maybe the Open Source team

was pushing for more GitHub integration or something? I really have no idea.

• Anyway, repository tailers do everything these could do (and much more).

Test Plan:

• Ran tailers off GitHub for many months without needing post-receive hooks.
• Grepped for relevant strings, couldn't find any references.
• Used "Repository" edit interface for a Git repository.

Reviewers: btrahan, jungejason

Reviewed By: jungejason

CC: aran, jungejason

Maniphest Tasks: T706

Differential Revision: https://secure.phabricator.com/D1273