GH-125413: Revert addition of `pathlib.Path.scandir()` method (#127377)

Remove documentation for `pathlib.Path.scandir()`, and rename the method to
`_scandir()`. In the private pathlib ABCs, make `iterdir()` abstract and
call it from `_scandir()`.

It's not worthwhile to add this method at the moment - see discussion:
https://discuss.python.org/t/ergonomics-of-new-pathlib-path-scandir/71721

Co-authored-by: Steve Dower <steve.dower@microsoft.com>