Prune never executed code

Summary:
GitHubGateController::onPost contains an abort instruction.

This throws an exception, so return isn't needed.

This has been noticed look at the D213 test coverage.

Test Plan:

• Fire a GitHub valid payload
• Fire a GitHub invalid payload
• phpunit

Reviewers: dereckson

Subscribers: alken-orin

Differential Revision: http://devcentral.nasqueron.org/D214