Elements. Remove AnnotatableElement.

Now that ManifestItem(s) store only ElementImpl, we don't need it.

Change-Id: I911fb22382aa73556ee65b291d30009288851185
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/425143
Reviewed-by: Paul Berry <paulberry@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

pkg/analyzer/lib/src/dart/element/element.dart

@@ -63,14 +63,11 @@ import 'package:pub_semver/pub_semver.dart';
 // TODO(scheglov): https://github.com/dart-lang/dartdoc/issues/4039
 typedef ConstructorElementImpl = ConstructorFragmentImpl;
 
-abstract class AnnotatableElement implements Element, Annotatable {
+abstract class AnnotatableElementImpl implements ElementImpl2, Annotatable {
   @override
   MetadataImpl get metadata2;
 }
 
-abstract class AnnotatableElementImpl
-    implements ElementImpl2, AnnotatableElement {}
-
 /// Shared implementation for an augmentable [Fragment].
 mixin AugmentableFragment on FragmentImpl {
   bool get isAugmentation {
@@ -2276,14 +2273,16 @@ class EnumFragmentImpl extends InterfaceFragmentImpl implements EnumFragment {
 
 /// Common base class for all analyzer-internal classes that implement
 /// `ExecutableElement2`.
-abstract class ExecutableElement2OrMember
-    implements ExecutableElement, AnnotatableElement {
+abstract class ExecutableElement2OrMember implements ExecutableElement {
   @override
   ExecutableElementImpl2 get baseElement;
 
   @override
   List<FormalParameterElementMixin> get formalParameters;
 
+  @override
+  MetadataImpl get metadata2;
+
   @override
   TypeImpl get returnType;