Remove getLink() from ArcanistLinterTestCase

Summary:
Installations extend this.

Another solution would be to extend ArcanistLinterTestCase from ArcanistArcanistLinterTestCase and return null in getLink() to avoid code duplication but I prefer clean class hierarchy.

Test Plan:

$ arc unit

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3878