Remove Protobuf load/export feature

[hcho3]

I'm about to refactor the whole codebase to accommodate multiple data types in tree models (#189 (comment)), and the current Protobuf spec is deficient in accommodating multiple data types.

Since supporting LightGBM model is a higher priority than Protobuf support, I am removing Protobuf load/export feature for the time being.

[codecov-commenter]

Codecov Report

Merging #193 into mainline will increase coverage by 0.04%.
The diff coverage is n/a.

@@              Coverage Diff               @@
##             mainline     #193      +/-   ##
==============================================
+ Coverage       82.15%   82.20%   +0.04%     
+ Complexity         56       54       -2     
==============================================
  Files              86       85       -1     
  Lines            5890     5625     -265     
  Branches           48       48              
==============================================
- Hits             4839     4624     -215     
+ Misses           1022      973      -49     
+ Partials           29       28       -1     

Impacted Files	Coverage Δ	Complexity Δ
include/treelite/frontend.h	50.00% <ø> (ø)	0.00 <0.00> (ø)
python/treelite/frontend.py	73.42% <ø> (-0.37%)	0.00 <0.00> (ø)
src/c_api/c_api.cc	75.96% <ø> (-0.76%)	0.00 <0.00> (ø)
src/compiler/ast/ast.h	36.90% <ø> (ø)	0.00 <0.00> (ø)
...c/main/java/ml/dmlc/treelite4j/java/Predictor.java	59.25% <0.00%> (-3.71%)	17.00% <0.00%> (-2.00%)
include/treelite/tree_impl.h	88.94% <0.00%> (-0.27%)	0.00% <0.00%> (ø%)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 70bdebe...c918824. Read the comment docs.