Remove PhabricatorSearchEngineTestCase

Summary:
Ref T12450. This is now pointless and just asserts that cluster.search has a default value.

We might restore a fancier version of this eventually, but get rid of this for now.

Test Plan: Scruitinized the test case.

Reviewers: chad, 20after4

Reviewed By: 20after4

Maniphest Tasks: T12450

Differential Revision: https://secure.phabricator.com/D17573