DOC: update whatsnew 0.23.1
One of the last commits apparently added back the subsections
ah, @jreback I see that you actually added that commit to that PR explicitly (I assumed is was due to some rebase or so). I removed the subsections on purpose in https://github.com/pandas-dev/pandas/pull/21368, because IMO for such a short list it is not really needed and rather adds noise to the toc (but can certainly be discussed)