cleanup: pk_is_clustering_key() -> is_clustering_key()

where PK is neither required nor implied

sql/handler.cc

@@ -2813,7 +2813,7 @@ double handler::keyread_time(uint index, uint ranges, ha_rows rows)
 {
   DBUG_ASSERT(ranges == 0 || ranges == 1);
   size_t len= table->key_info[index].key_length + ref_length;
-  if (table->file->pk_is_clustering_key(index))
+  if (table->file->is_clustering_key(index))
     len= table->s->stored_rec_length;
   double cost= (double)rows*len/(stats.block_size+1)*IDX_BLOCK_COPY_COST;
   if (ranges)

sql/multi_range_read.cc

@@ -385,7 +385,7 @@ ha_rows handler::multi_range_read_info(uint keyno, uint n_ranges, uint n_rows,
 
   cost->reset();
   /* Produce the same cost as non-MRR code does */
-  if (!pk_is_clustering_key(keyno))
+  if (!is_clustering_key(keyno))
   {
     /*
       idx_io_count could potentially be increased with the number of