[Breaking] Remove the use of dmlc parsers

[hcho3]

Extracted from #285.

• Do not use dmlc/data.h.
• [Breaking] Drop support for loaindg DMatrix from text files in LIBSVM or CSV format. Users are asked to use 3rd-party libraries such as NumPy, Pandas, and scikit-learn to load data from text files. The C API function TreeliteDMatrixCreateFromFile is removed.
• Python tests now require scikit-learn package, in order to load test data from LIBSVM files.

[codecov]

Codecov Report

Merging #290 (79dbbbc) into mainline (5329e85) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@              Coverage Diff               @@
##             mainline     #290      +/-   ##
==============================================
- Coverage       84.78%   84.77%   -0.02%     
  Complexity         46       46              
==============================================
  Files             102      102              
  Lines            7902     7821      -81     
  Branches           50       50              
==============================================
- Hits             6700     6630      -70     
+ Misses           1177     1166      -11     
  Partials           25       25              

Impacted Files	Coverage Δ
include/treelite/data.h	100.00% <ø> (+16.66%)	⬆️
include/treelite/predictor.h	92.30% <ø> (ø)
python/treelite/util.py	83.87% <ø> (+2.78%)	⬆️
runtime/python/treelite_runtime/predictor.py	85.86% <0.00%> (-0.77%)	⬇️
src/annotator.cc	90.74% <ø> (ø)
src/c_api/c_api.cc	96.65% <ø> (ø)
src/c_api/c_api_common.cc	100.00% <ø> (ø)
src/compiler/ast_native.cc	96.08% <ø> (ø)
src/data.cc	80.76% <ø> (-2.18%)	⬇️
src/gtil/predict.cc	85.50% <ø> (ø)
... and 1 more

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