Fix failing unit tests for ArcanistLesscLinter
Summary: A bunch of unit tests are failing with the latest version of lessc (v2.3.0). I decided to delete a bunch of test cases for this linter as we have far too many at the moment.
Test Plan: arc unit
Reviewers: chad, Blessed Reviewers, epriestley
Reviewed By: Blessed Reviewers, epriestley
Subscribers: Korvin, epriestley
Differential Revision: https://secure.phabricator.com/D11521