Remove the `PhutilLintEngine` class.

Summary: After D9057 and D9064 this class is no longer used.

Test Plan: N/A

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9197