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