Don't support severity customization in pyflakes

Summary: Pyflakes doesn't have any message codes, so we can't do anything with severities.

Test Plan: Ran `arc lint` with severities on one of these linters, got an error.

Reviewers: joshuaspence, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D9222