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