remove unnecessary allocations in HistogramSumReducer

[jameslamb]

Contributes to #4705.

Int32HistogramSumReducer and Int16HistogramSumReducer allocate an int and assign to it the value of OMP_NUM_THREADS(), then only use that value immediately after, for one OpenMP for loop.

This PR proposes just directly passing OMP_NUM_THREADS() to the configuration for those OpenMP for loop pragmas, to avoid the unnecessary allocation.

This also resolves the following compiler warnings that show up on clang when OpenMP support is disabled:

LightGBM/include/LightGBM/bin.h:66:13: warning: unused variable 'num_threads' [-Wunused-variable]
  const int num_threads = OMP_NUM_THREADS();
            ^
LightGBM/include/LightGBM/bin.h:77:13: warning: unused variable 'num_threads' [-Wunused-variable]
  const int num_threads = OMP_NUM_THREADS();
            ^
2 warnings generated.

How I tested this

LightGBM's CI covers the cases where OpenMP is available. I ran the following locally to confirm that this suppresses the compiler warnings when building without OpenMP support.

rm -rf ./build
mkdir ./build
cd ./build
cmake -DUSE_OPENMP=off ..
make -j3 _lightgbm

[github-actions]

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