CLEAN: Removing outdated and unused CI files
Removing outdated and unused files.
- `ci/print_versions.py` doesn't work, as the used path has changed. It's not used anywhere in the code, and when needed, `python -c "import pandas; pandas.show_versions();"` is used instead.
- `ci/README.txt` it's outdated and incomplete. After updating the CI and making it more compact, I'll try to add a section to the contributing page in the docs, about what we have. This should be more useful than this file.