Deprecate C codegen

[hcho3]

Deprecate C codegen part of Treelite. Direct users to use TL2cgen instead.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.88 ⚠️

Comparison is base (4c48e16) 85.34% compared to head (af7074b) 84.47%.

❗ Current head af7074b differs from pull request most recent head b8b053e. Consider uploading reports for the commit b8b053e to get more accurate results

Additional details and impacted files

@@              Coverage Diff               @@
##             mainline     #493      +/-   ##
==============================================
- Coverage       85.34%   84.47%   -0.88%     
  Complexity         42       42              
==============================================
  Files             112      113       +1     
  Lines            9910     8868    -1042     
  Branches          617       40     -577     
==============================================
- Hits             8458     7491     -967     
+ Misses           1429     1354      -75     
  Partials           23       23              

Impacted Files	Coverage Δ
python/treelite/annotator.py	84.37% <100.00%> (+1.04%)	⬆️
python/treelite/contrib/__init__.py	69.84% <100.00%> (-5.57%)	⬇️
python/treelite/frontend.py	78.12% <100.00%> (+0.23%)	⬆️
python/treelite/warn.py	100.00% <100.00%> (ø)
runtime/python/treelite_runtime/predictor.py	86.52% <100.00%> (-0.32%)	⬇️
runtime/python/treelite_runtime/warn.py	100.00% <100.00%> (ø)

... and 66 files with indirect coverage changes

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