Remove ancient "PhabricatorQuickSearchEngineExtension" compatibility class

Summary: Ref T5378. This class was renamed more than a year ago, in D19087. Remove the leftover compatiblity layer.

Test Plan: grep

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T5378

Differential Revision: https://secure.phabricator.com/D20509