Remove ReturnData::nx, ReturnData::nxtrue

[dweindl]

Replacements ReturnData::nx_rdata and ReturnData::nxtrue_rdata have been installed long ago. Let's get rid of the aliases.

[codecov]

Codecov Report

❌ Patch coverage is 39.58333% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.61%. Comparing base (5e0acb2) to head (777c2bb).
⚠️ Report is 122 commits behind head on main.

Files with missing lines	Patch %	Lines
src/rdata.cpp	72.22%	5 Missing ⚠️
models/model_calvetti_py/main.cpp	0.00%	3 Missing ⚠️
models/model_dirac_py/main.cpp	0.00%	3 Missing ⚠️
models/model_events_py/main.cpp	0.00%	3 Missing ⚠️
models/model_jakstat_adjoint_py/main.cpp	0.00%	3 Missing ⚠️
models/model_nested_events_py/main.cpp	0.00%	3 Missing ⚠️
models/model_neuron_py/main.cpp	0.00%	3 Missing ⚠️
models/model_robertson_py/main.cpp	0.00%	3 Missing ⚠️
models/model_steadystate_py/main.cpp	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2966      +/-   ##
==========================================
- Coverage   77.29%   76.61%   -0.69%     
==========================================
  Files         307      307              
  Lines       19912    19910       -2     
  Branches     1502     1502              
==========================================
- Hits        15391    15254     -137     
- Misses       4508     4643     +135     
  Partials       13       13              

Flag	Coverage Δ
cpp	72.77% <39.58%> (-0.01%)	⬇️
cpp_python	38.29% <22.91%> (-0.01%)	⬇️
petab	39.35% <50.00%> (-0.01%)	⬇️
python	70.32% <39.58%> (-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
include/amici/rdata.h	100.00% <ø> (ø)
include/amici/serialization.h	95.17% <100.00%> (ø)
python/sdist/amici/numpy.py	87.82% <ø> (ø)
src/hdf5.cpp	85.68% <100.00%> (ø)
models/model_calvetti_py/main.cpp	0.00% <0.00%> (ø)
models/model_dirac_py/main.cpp	0.00% <0.00%> (ø)
models/model_events_py/main.cpp	0.00% <0.00%> (ø)
models/model_jakstat_adjoint_py/main.cpp	0.00% <0.00%> (ø)
models/model_nested_events_py/main.cpp	0.00% <0.00%> (ø)
models/model_neuron_py/main.cpp	0.00% <0.00%> (ø)
... and 3 more

... and 6 files with indirect coverage changes

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