Remove unused null_count field from OrcColumnStatistics (apache#146)

The null_count field was always hardcoded to -1 and never consumed.
ORC statistics provide hasNull (boolean) but not a direct null count.
Remove the dead field rather than keeping it for speculative future use.

Author: cbb330

cpp/src/arrow/adapters/orc/adapter.cc

@@ -110,7 +110,6 @@ Result<OrcColumnStatistics> ConvertColumnStatistics(
 
   stats.has_null = orc_stats->hasNull();
   stats.num_values = static_cast<int64_t>(orc_stats->getNumberOfValues());
-  stats.null_count = -1;  // ORC doesn't provide null_count, only hasNull
   stats.has_min_max = false;
   stats.min = nullptr;
   stats.max = nullptr;

cpp/src/arrow/adapters/orc/adapter.h

@@ -43,8 +43,6 @@ namespace orc {
 struct OrcColumnStatistics {
   /// \brief Whether the column contains null values
   bool has_null;
-  /// \brief Number of null values in the column (-1 if not available)
-  int64_t null_count;
   /// \brief Total number of values in the column
   int64_t num_values;
   /// \brief Whether min/max statistics are available