fix a warning.

src/c_api.cpp

@@ -561,7 +561,7 @@ int LGBM_DatasetCreateFromCSR(const void* indptr,
       auto idx = sample_indices[i];
       auto row = get_row_fun(static_cast<int>(idx));
       for (std::pair<int, double>& inner_data : row) {
-        CHECK(static_cast<size_t>(inner_data.first) < num_col);
+        CHECK(inner_data.first < num_col);
         if (std::fabs(inner_data.second) > kZeroThreshold || std::isnan(inner_data.second)) {
           sample_values[inner_data.first].emplace_back(inner_data.second);
           sample_idx[inner_data.first].emplace_back(static_cast<int>(i));