Clean up symbolic_functions.{cpp,h}

[dweindl]

Remove MATLAB leftovers. Clean up.

Related to #2727.

[codecov]

Codecov Report

❌ Patch coverage is 85.18519% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.45%. Comparing base (570c71d) to head (4f56ebe).
⚠️ Report is 129 commits behind head on main.

Files with missing lines	Patch %	Lines
src/solver.cpp	87.50%	2 Missing ⚠️
src/symbolic_functions.cpp	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2956      +/-   ##
==========================================
- Coverage   77.12%   76.45%   -0.68%     
==========================================
  Files         307      307              
  Lines       19947    19925      -22     
  Branches     1508     1502       -6     
==========================================
- Hits        15385    15234     -151     
- Misses       4549     4678     +129     
  Partials       13       13              

Flag	Coverage Δ
cpp	72.61% <85.18%> (-0.01%)	⬇️
cpp_python	38.22% <55.55%> (+0.05%)	⬆️
petab	39.44% <29.16%> (+0.05%)	⬆️
python	70.36% <66.66%> (+<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/edata.h	100.00% <ø> (ø)
src/edata.cpp	88.75% <100.00%> (ø)
src/rdata.cpp	82.57% <100.00%> (ø)
src/solver.cpp	80.87% <87.50%> (ø)
src/symbolic_functions.cpp	91.30% <33.33%> (-2.03%)	⬇️

... and 7 files with indirect coverage changes

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