Remove unused extension method, replaced in analyzer 8.4.0

Author: srawlins

lib/src/model/model_element.dart

@@ -831,16 +831,6 @@ extension on Element {
   }
 }
 
-// Copied from analyzer's `lib/src/dart/element/extensions.dart`. Re-use that
-// extension if it becomes public.
-extension on Element {
-  /// Whether this Element is annotated with a `Deprecated` annotation with a
-  /// `_DeprecationKind` of [kind].
-  bool isDeprecatedWithKind(String kind) => metadata.annotations
-      .where((e) => e.isDeprecated)
-      .any((e) => e.deprecationKind == kind);
-}
-
 // Copied from analyzer's `lib/src/dart/element/extensions.dart`. Re-use that
 // extension if it becomes public.
 extension ElementAnnotationExtension on ElementAnnotation {