Remove numba backend

[jessegrabowski]

Remove the backend="numba" option. It was causing needless maintenance and testing burden. We can still compile to numba via pytensor, using mode="NUMBA"

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📚 Documentation preview 📚: https://gEconpy--55.org.readthedocs.build/en/55/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.29%. Comparing base (3e9c9cf) to head (28597eb).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #55      +/-   ##
==========================================
- Coverage   75.48%   75.29%   -0.20%     
==========================================
  Files          33       33              
  Lines        4936     4857      -79     
==========================================
- Hits         3726     3657      -69     
+ Misses       1210     1200      -10     

Files with missing lines	Coverage Δ
gEconpy/model/compile.py	100.00% <100.00%> (ø)
gEconpy/model/model.py	85.73% <100.00%> (ø)
gEconpy/model/parameters.py	100.00% <ø> (ø)
gEconpy/model/steady_state.py	73.87% <ø> (ø)
gEconpy/numbaf/utilities.py	39.79% <ø> (-19.60%)	⬇️
gEconpy/solvers/cycle_reduction.py	91.96% <100.00%> (ø)

... and 2 files with indirect coverage changes

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