API: Remove integer position args from xy for plotting
- [x] closes #20056 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [?] whatsnew entry

This is based on discussion from #20000 where we decided it was confusing to allow `df.plot` to support both labels AND positions. I wasn't sure where to put this in whatsnew, so lmk and I'll update it.