Remove openssh dependency as soon as this is possible

[BenjaminRodenberg]

This issue is mainly a reminder.

In #76 we added openssh to the spack package.

As soon as spack/spack#22115 is merged, we will be able to remove openssh again.

[haampie]

For me openmpi tests without system openssh work now: https://github.com/haampie-spack/ci-example

I also updated my example to just use spack -e ./tools/environments/ci install -v --test=root which now works well after spack/spack#22040 was merged.

[BenjaminRodenberg]

Thanks for the notice @haampie. I'm testing this in #95 and will merge it as soon as the test are successful.