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