CLN: remove kwargs in Index.format
Removes kwargs in ``Index.format`` and subclasses. Also changes ``Index._format_with_header`` to have parameter ``na_rep`` not have a default value (in order to not have default value set in two locations).

Related to #35118.
Hello @topper-123! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:













There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-07-14 20:50:39 UTC Updated. @WillAyd? Ping. I missed that, I've updated again. thanks @topper-123 