Remove more references to the old plugin system

pkg/analysis_server/lib/src/socket_server.dart

Index: pkg/analysis_server/lib/src/socket_server.dart
diff --git a/pkg/analysis_server/lib/src/socket_server.dart b/pkg/analysis_server/lib/src/socket_server.dart
index f766bab24524ca97c96bca7986fbd48f67d1696a..d4ef8f040a41e85af7dc57ed6030d2c122e2fe5c 100644
--- a/pkg/analysis_server/lib/src/socket_server.dart
+++ b/pkg/analysis_server/lib/src/socket_server.dart
@@ -12,7 +12,6 @@ import 'package:analyzer/instrumentation/instrumentation.dart';
 import 'package:analyzer/plugin/resolver_provider.dart';
 import 'package:analyzer/source/pub_package_map_provider.dart';
 import 'package:analyzer/src/generated/sdk.dart';
-import 'package:plugin/plugin.dart';
 
 /**
  * Instances of the class [SocketServer] implement the common parts of
@@ -40,11 +39,6 @@ class SocketServer {
    */
   AnalysisServer analysisServer;
 
-  /**
-   * The plugins that are defined outside the analysis_server package.
-   */
-  List<Plugin> userDefinedPlugins;
-
   SocketServer(
       this.analysisServerOptions,
       this.sdkManager,
@@ -90,6 +84,5 @@ class SocketServer {
         diagnosticServer: diagnosticServer,
         fileResolverProvider: fileResolverProvider,
         packageResolverProvider: packageResolverProvider);
-    analysisServer.userDefinedPlugins = userDefinedPlugins;
   }
 }