Disable the customization of linter severities for ArcanistCommitLinter
Summary: The expectation is that this linter only raises errors.
Test Plan: This is mostly theoretical.
Reviewers: epriestley, Blessed Reviewers
Reviewed By: epriestley, Blessed Reviewers
Subscribers: Korvin, epriestley
Differential Revision: https://secure.phabricator.com/D11331