(maint) Remove unused scope from closure call_by_name

The big removal of 3x parser/ast missed to remove the unused scope
that was present in Closure#call_by_name. This PR is a follow up on PUP-3274.

This commit removes the unused scope, and corrects a typo in code that
supports use cases not yet merged to master (if invoked would cause a
hard stop on undefined variable 'scope').

The documentation comment is also updated to reflect that scope is no longer given in the API.
Yes, I think it helps.