STYLE: Remove unused private member m_PointsStartPosition from OBJMeshIO

Note that the member variable was already there, unused, with the initial
OBJMeshIO commit, 9403e3b, "ENH: Add mesh IO"
by Wanlin Zhu, 10 Aug 2011.

The similar OFFMeshIO class also has a m_PointsStartPosition member, but
OFFMeshIO _does_ use it!

Co-authored-by: greptile-apps[bot] <165735046+greptile-apps[bot]@users.noreply.github.com>

Author: N-Dekker

Modules/IO/MeshOBJ/include/itkOBJMeshIO.h

@@ -181,9 +181,6 @@ class ITKIOMeshOBJ_EXPORT OBJMeshIO : public MeshIOBase
 private:
   [[nodiscard]] std::ifstream
   OpenFile() const;
-
-  std::streampos m_PointsStartPosition{}; // file position for points relative to
-                                          // std::ios::beg
 };
 } // end namespace itk