CLN: OrderedDict->dict
- [x] closes #30469 
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Grepping for OrderedDict in *.py files I see 84 more usages, but AFAICT these are all explicitly checking for the class, so I'm considering this as closing #30469.