(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.
Sorry, this paragraph is scrambling my brain.  'when the scope given to it when initialized goes "out of scope".'  Oh, okay, I think I'm getting it.  How about:

'it is not safe to use this closure once the scope it was given during initialization has gone "out of scope".'

I think this is makes it clearer, although the question of when or why a scope goes 'out of scope' remains.