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