Revert "Resurrect pandas 0.17.1"

This reverts commit 96874c6e.

- No package depends on this version.
- As a guideline we only keep one version of each package. Exceptions exist but only with a good reason.
- Expressions shouldn't be in `python-packages.nix` but in `python-modules`, as explained in the docs and the header of the file.
- I won't maintain these versions.