Remove hook functionality

Summary: Fixes T7674. Remove remaining commit hook functionality.

Test Plan: Unit tests still pass?

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7674

Differential Revision: https://secure.phabricator.com/D12698