Remove versions from setup.py

[tovogt]

@emanuel-schmid We should make sure that the version numbers in setup.py fit with the version numbers in the requirements yml-files because, otherwise, pip install -e . will change the versions again. I even suggest to remove the version numbers from setup.py altogether.

Note that, contrary to the CLIMADA docs, there seems to be a general recommendation to use pip install -e instead of conda develop: conda/conda-build#2633