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