DEPR: to_perioddelta
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

The only usage was in liboffsets and that has now been removed.
sure, pls add to the deprecation removal issue > sure, pls add to the deprecation removal issue

done