Disable the customization of linter severities for `ArcanistCommitLinter`
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

julkiewicz/arcanist,julkiewicz/arcanist,julkiewicz/arcanist,julkiewicz/arcanist