Remove LinkedNodeVariablesDeclaration.

We don't need it anymore - we can go up the AST hierarchy to the
enclosing TopLevelVariableDeclaration.

[email protected], [email protected]

Change-Id: Iadc9985bb5bcaa465c3505794ceac51e7b06a930
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/103564
Reviewed-by: Brian Wilkerson <[email protected]>
Reviewed-by: Paul Berry <[email protected]>

Author: scheglov