Remove pre-commit hook documentation
Summary: Ref T7674. This functionality has been removed.
Test Plan: Eyeball it.
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: Korvin, epriestley
Maniphest Tasks: T7674
Differential Revision: https://secure.phabricator.com/D12713