Remove auto-generated `flux_r0` expression in SBML models without reactions

When importing SBML models without reactions, the amici model will always contain an `flux_r0` expression.
This is confusing and should be easily avoidable.

Introduced here:
https://github.com/AMICI-dev/AMICI/blob/d8c41f3d9d0e6089d35efddf05a1a9c773a704f5/python/sdist/amici/importers/sbml/__init__.py#L1359