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