remove unused variable

std::streampos position is never used.

Author: lulyon

hnswlib/hnswalg.h

@@ -684,7 +684,6 @@ class HierarchicalNSW : public AlgorithmInterface<dist_t> {
 
     void saveIndex(const std::string &location) {
         std::ofstream output(location, std::ios::binary);
-        std::streampos position;
 
         writeBinaryPOD(output, offsetLevel0_);
         writeBinaryPOD(output, max_elements_);