Remove unused argument dwdx in fdydx

[dweindl]

Remove unused and unsafe argument dwdx in fdydx.

Related to #2727.

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.61%. Comparing base (5e0acb2) to head (4e66568).
⚠️ Report is 122 commits behind head on main.

Files with missing lines	Patch %	Lines
models/model_calvetti_py/model_calvetti_py.h	0.00%	2 Missing ⚠️
models/model_calvetti_py/dydx.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2967      +/-   ##
==========================================
- Coverage   77.29%   76.61%   -0.69%     
==========================================
  Files         307      307              
  Lines       19912    19912              
  Branches     1502     1502              
==========================================
- Hits        15391    15255     -136     
- Misses       4508     4644     +136     
  Partials       13       13              

Flag	Coverage Δ
cpp	72.76% <88.88%> (-0.01%)	⬇️
cpp_python	38.29% <11.11%> (-0.01%)	⬇️
petab	39.35% <100.00%> (-0.01%)	⬇️
python	70.32% <88.88%> (-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/abstract_model.h	100.00% <ø> (ø)
models/model_dirac_py/dydx.cpp	100.00% <100.00%> (ø)
models/model_dirac_py/model_dirac_py.h	58.44% <100.00%> (ø)
models/model_events_py/dydx.cpp	100.00% <100.00%> (ø)
models/model_events_py/model_events_py.h	65.89% <100.00%> (ø)
models/model_jakstat_adjoint_py/dydx.cpp	100.00% <100.00%> (ø)
...odel_jakstat_adjoint_py/model_jakstat_adjoint_py.h	69.87% <100.00%> (ø)
models/model_nested_events_py/dydx.cpp	100.00% <100.00%> (ø)
...ls/model_nested_events_py/model_nested_events_py.h	62.89% <100.00%> (ø)
models/model_neuron_py/dydx.cpp	100.00% <100.00%> (ø)
... and 11 more

... and 6 files with indirect coverage changes

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