Remove the `ExampleLintEngine` class.

Summary: Ref T2039. Now that the `ArcanistConfigurationDrivenLintEngine` has gained more widespread use, we do no longer expect users to need to write their own lint engine. The documentation has already been updated to reflect this.

Test Plan: N/A

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T2039

Differential Revision: https://secure.phabricator.com/D9198