Make the PhutilLintEngine final.

Summary:
Phabricator now uses the ArcanistConfigurationDrivenLintEngine lint
engine instead of the PhabricatorLintEngine lint engine.

Depends on D9064.

Test Plan: N/A

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9069