test: remove neural network tests, clean up envs

[gdalle]

• Remove tests on Lux and Flux scenarios since neither is using or recommending DI to compute gradients
• Clean up DI's test env so that DIT is natively inside it

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.11%. Comparing base (9edc437) to head (4d12efb).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #914       +/-   ##
===========================================
+ Coverage   76.36%   98.11%   +21.74%     
===========================================
  Files         131      132        +1     
  Lines        7964     7959        -5     
===========================================
+ Hits         6082     7809     +1727     
+ Misses       1882      150     -1732     

Flag	Coverage Δ
DI	98.85% <ø> (+30.47%)	⬆️
DIT	96.19% <ø> (-0.20%)	⬇️

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[gdalle]

Running into JuliaLang/Pkg.jl#1585 again