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

src/__phutil_library_map__.php

@@ -4313,7 +4313,6 @@ phutil_register_library_map(array(
     'PhabricatorQueryOrderItem' => 'infrastructure/query/order/PhabricatorQueryOrderItem.php',
     'PhabricatorQueryOrderTestCase' => 'infrastructure/query/order/__tests__/PhabricatorQueryOrderTestCase.php',
     'PhabricatorQueryOrderVector' => 'infrastructure/query/order/PhabricatorQueryOrderVector.php',
-    'PhabricatorQuickSearchEngineExtension' => 'applications/search/engineextension/PhabricatorQuickSearchEngineExtension.php',
     'PhabricatorRateLimitRequestExceptionHandler' => 'aphront/handler/PhabricatorRateLimitRequestExceptionHandler.php',
     'PhabricatorRebuildIndexesWorker' => 'applications/search/worker/PhabricatorRebuildIndexesWorker.php',
     'PhabricatorRecaptchaConfigOptions' => 'applications/config/option/PhabricatorRecaptchaConfigOptions.php',
@@ -10552,7 +10551,6 @@ phutil_register_library_map(array(
       'Phobject',
       'Iterator',
     ),
-    'PhabricatorQuickSearchEngineExtension' => 'PhabricatorDatasourceEngineExtension',
     'PhabricatorRateLimitRequestExceptionHandler' => 'PhabricatorRequestExceptionHandler',
     'PhabricatorRebuildIndexesWorker' => 'PhabricatorWorker',
     'PhabricatorRecaptchaConfigOptions' => 'PhabricatorApplicationConfigOptions',

src/applications/search/engineextension/PhabricatorQuickSearchEngineExtension.php

@@ -1,6 +0,0 @@
-<?php
-
-// TODO: This is an older name "PhabricatorDatasourceEngineExtension" purely
-// to preserve compatibility that should be removed soon.
-abstract class PhabricatorQuickSearchEngineExtension
-  extends PhabricatorDatasourceEngineExtension {}