Remove "_-_" -> "-" slug behavior

Summary: Fixes T9573. This incorrectly affected Phriction. I could restore it for only projects, but you didn't like the rule very much anyway and I don't feel strongly about it.

Test Plan: Unit tests.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9573

Differential Revision: https://secure.phabricator.com/D14287