Remove `debug` key from `.arclint` file format.

Summary: Having a `debug` key in the `.arclint` file format doesn't seem right. Instead, it would be better to just use a `PhutilConsole` and the `writeLog` method so that "debug" messages are output when using `arc --trace`.

Test Plan: Ran `arc lint --trace` in a repository using `.arclint`.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9021