Remove uses of the old plugin model from the analysis server

pkg/analysis_server/lib/plugin/analysis/occurrences/occurrences.dart

-// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
-// for details. All rights reserved. Use of this source code is governed by a
-// BSD-style license that can be found in the LICENSE file.
-
-/**
- * Support for client code that extends the analysis server by adding new
- * occurrences contributors.
- *
- * Plugins can register occurrences contributors. The registered contributors
- * will be used to get occurrence information any time the server is about to
- * send an 'analysis.occurrences' notification.
- *
- * If a plugin wants to add occurrence information, it should implement the
- * class [OccurrencesContributor] and then register the contributor by including
- * code like the following in the plugin's registerExtensions method:
- *
- *     @override
- *     void registerExtensions(RegisterExtension registerExtension) {
- *       ...
- *       registerExtension(
- *           OCCURRENCES_CONTRIBUTOR_EXTENSION_POINT_ID,
- *           new MyOccurrencesContributor());
- *       ...
- *     }
- */
-import 'package:analysis_server/plugin/analysis/occurrences/occurrences_core.dart';
-import 'package:analysis_server/src/plugin/server_plugin.dart';
-import 'package:plugin/plugin.dart';
-
-/**
- * The identifier of the extension point that allows plugins to register
- * occurrence information. The object used as an extension must be an
- * [OccurrencesContributor].
- */
-final String OCCURRENCES_CONTRIBUTOR_EXTENSION_POINT_ID = Plugin.join(
-    ServerPlugin.UNIQUE_IDENTIFIER,
-    ServerPlugin.OCCURRENCES_CONTRIBUTOR_EXTENSION_POINT);