Remove severity options from some linters.

Summary: The `ArcanistGeneratedLinter` and `ArcanistNoLintLinter` don't actually ever raise any linter messages, so it doesn't make sense to set custom severities for these linters. Instead, don't expose this configuration.

Test Plan: N/A

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9038