CI: Remove PyPy references in CI testing
We haven't had reliable PyPy testing in many years now and no one to champion supporting this platform. We only have 1 job that builds pandas with PyPy on Python 3.9 (already dropped). It's made more difficult that conda-forge no longer supports PyPy either, https://conda-forge.org/news/2024/08/14/sunsetting-pypy/ 

I don't think it's worth using resources for this job anymore. pandas can still have code for PyPy compatibility for those wanting to support PyPy independently.
Thanks @mroeschke 