Spack testing obsolete

[fsimonis]

The tests of the python bindings to keep spack working involves many layers as spack used to install packages by calling setup.py as follows:

$ python setup.py build
$ python setup.py install --root=...

As the python bindings are native extensions of precice using cython, this involved additional steps that needed testing.

Spack changed to directly using pip install in https://github.com/spack/spack/pull/27798. Meaning that we no longer need to test setup.py calls and the installation of the python bindings becomes significantly simpler and more robust.

I suggest to:

• remove testing of setup.py calls which are anyhow deprecated.
• remove testing of spack builds in the CI, including the generation of the build image and the checked-in spack package files.
• remove the explicit checks for a pip environment in the setup.py.

This allows us to more aggressively push #208 and #217.