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