Elements. Remove InstanceElementImpl.nonSynthetic, it is never synthetic.

...and ElementImpl.nonSynthetic already returns `this`.

Change-Id: Icabd01d48b33e1993290f620d0fc6b5867e4292b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/446142
Reviewed-by: Paul Berry <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: scheglov

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

@@ -4009,9 +4009,6 @@ abstract class InstanceElementImpl extends ElementImpl
   @override
   String? get name3 => name;
 
-  @override
-  Element get nonSynthetic => isSynthetic ? enclosingElement : this as Element;
-
   @override
   AnalysisSessionImpl get session => library.session;