(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 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').
(maint) Improve comment about "scope state" and closure
Merge pull request #3329 from hlindberg/MAINT_remove_unused_scope_call_by_name

(maint) Remove unused scope from closure call_by_name