Don't visit children in LibraryElementSuggestionBuilder.visitExtensionElement()

Change-Id: Ie90ff19737444f7b8ca3d17f0bf3b113bccb6f2d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/241301
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
diff --git a/pkg/analysis_server/lib/src/services/completion/dart/local_library_contributor.dart b/pkg/analysis_server/lib/src/services/completion/dart/local_library_contributor.dart
index ae44d1b..d852030 100644
--- a/pkg/analysis_server/lib/src/services/completion/dart/local_library_contributor.dart
+++ b/pkg/analysis_server/lib/src/services/completion/dart/local_library_contributor.dart
@@ -77,7 +77,6 @@
     if (opType.includeReturnValueSuggestions) {
       builder.suggestExtension(element, kind: kind, prefix: prefix);
     }
-    element.visitChildren(this);
   }
 
   @override