Remove the commented-out lines concerning `callable_iterator` in `_collections_abc.py`

[jonathandung]

Perhaps to prevent future confusion we can remove the commented-out lines about callable_iterator.

Originally posted by @JelleZijlstra in #92510 (comment)

Linked PRs

• gh-152148: Eliminate commented-out code referencing callable_iterator in collections.abc module source #152149