Remove AugmentableElement and FunctionElementImpl.enclosingElement2

These were left because DartDoc was not yet fully migrated.
But it did migrate since then.

Change-Id: Iae0ab451ffbd79d9370cf6c3f5da458f50f4fe06
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/423840
Commit-Queue: Konstantin Shcheglov <[email protected]>
Reviewed-by: Paul Berry <[email protected]>

Author: scheglov

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

@@ -73,11 +73,6 @@ abstract class AnnotatableElement implements Element2, Annotatable {
 abstract class AnnotatableElementImpl
     implements ElementImpl2, AnnotatableElement {}
 
-@Deprecated('This is an internal class, do not use it')
-// TODO(scheglov): remove it when DartDoc stops using it
-// https://github.com/dart-lang/dartdoc/issues/4015
-mixin AugmentableElement on ElementImpl2 {}
-
 /// Shared implementation for an augmentable [Fragment].
 mixin AugmentableFragment on ElementImpl {
   bool get isAugmentation {
@@ -4736,10 +4731,6 @@ sealed class FunctionElementImpl extends ExecutableElementImpl
   @override
   ExecutableElementImpl get declaration => this;
 
-  @Deprecated('ElementImpl should have never implemented Element2')
-  // TODO(scheglov): remove after https://github.com/dart-lang/dartdoc/issues/4017
-  Element2? get enclosingElement2 => null;
-
   @override
   Fragment? get enclosingFragment {
     switch (enclosingElement3) {