Remove auto-generated flux_r0 expression in models without reactions

[dweindl]

This may have been necessary with earlier sympy versions, but I don't think we still need that. Let's not make users wonder where that expression comes from.

Closes #3045.

[codecov]

Codecov Report

❌ Patch coverage is 38.09524% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.09%. Comparing base (c4a395b) to head (e157cd3).
⚠️ Report is 35 commits behind head on main.

Files with missing lines	Patch %	Lines
python/sdist/amici/importers/sbml/__init__.py	54.54%	5 Missing ⚠️
models/model_calvetti_py/model_calvetti_py.h	0.00%	1 Missing ⚠️
models/model_dirac_py/model_dirac_py.h	0.00%	1 Missing ⚠️
models/model_events_py/model_events_py.h	0.00%	1 Missing ⚠️
...odel_jakstat_adjoint_py/model_jakstat_adjoint_py.h	0.00%	1 Missing ⚠️
...ls/model_nested_events_py/model_nested_events_py.h	0.00%	1 Missing ⚠️
models/model_neuron_py/model_neuron_py.h	0.00%	1 Missing ⚠️
models/model_robertson_py/model_robertson_py.h	0.00%	1 Missing ⚠️
models/model_steadystate_py/model_steadystate_py.h	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3057      +/-   ##
==========================================
- Coverage   77.74%   77.09%   -0.66%     
==========================================
  Files         313      313              
  Lines       20661    20668       +7     
  Branches     1499     1499              
==========================================
- Hits        16062    15933     -129     
- Misses       4590     4726     +136     
  Partials        9        9              

Flag	Coverage Δ
cpp	71.18% <38.09%> (-0.01%)	⬇️
cpp_python	37.23% <38.09%> (-0.03%)	⬇️
petab	45.87% <61.53%> (-0.01%)	⬇️
petab_sciml	13.83% <38.46%> (+<0.01%)	⬆️
python	69.84% <38.09%> (-0.01%)	⬇️
sbmlsuite-jax	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
models/model_calvetti_py/dwdw.cpp	100.00% <ø> (ø)
models/model_calvetti_py/dxdotdw.cpp	100.00% <ø> (ø)
models/model_dirac_py/deltaqB.cpp	0.00% <ø> (ø)
models/model_dirac_py/dxdotdp_explicit.cpp	100.00% <ø> (ø)
models/model_dirac_py/dxdotdx_explicit.cpp	100.00% <ø> (ø)
models/model_dirac_py/xdot.cpp	100.00% <ø> (ø)
models/model_events_py/deltaqB.cpp	0.00% <ø> (ø)
models/model_events_py/deltaxB.cpp	0.00% <ø> (ø)
models/model_events_py/dxdotdp_explicit.cpp	100.00% <ø> (ø)
models/model_events_py/dxdotdx_explicit.cpp	100.00% <ø> (ø)
... and 28 more

... and 4 files with indirect coverage changes

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[dweindl]

This may have been necessary with earlier sympy versions, but I don't think we still need that.

Almost. sympy.Matrix.row_insert doesn't play nice.

[dweindl]

The stored test results need updating because w changed. The rest is working.