Remove more references to the old plugin system

pkg/analysis_server/lib/starter.dart

Index: pkg/analysis_server/lib/starter.dart
diff --git a/pkg/analysis_server/lib/starter.dart b/pkg/analysis_server/lib/starter.dart
index 087074dc2f18ea6a4a640a284ece40dca00b602e..cca06dab3a88f03db23bb21bd5b6a4f565e044c0 100644
--- a/pkg/analysis_server/lib/starter.dart
+++ b/pkg/analysis_server/lib/starter.dart
@@ -6,7 +6,6 @@ import 'package:analysis_server/src/analysis_server.dart';
 import 'package:analysis_server/src/server/driver.dart';
 import 'package:analyzer/instrumentation/instrumentation.dart';
 import 'package:analyzer/plugin/resolver_provider.dart';
-import 'package:plugin/plugin.dart';
 
 /**
  * An object that can be used to start an analysis server. This class exists so
@@ -40,11 +39,6 @@ abstract class ServerStarter {
   void set packageResolverProvider(ResolverProvider provider);
 
   /**
-   * Set the [plugins] that are defined outside the analysis_server package.
-   */
-  void set userDefinedPlugins(List<Plugin> plugins);
-
-  /**
    * Use the given command-line [arguments] to start this server.
    *
    * At least temporarily returns AnalysisServer so that consumers of the