Remove duplicated serialization and deserialization of GCP list that was found in 4 instances in the code base... So now TPS and Polynomial GCP transformation have the Z component of their GCPs properly serialized/deserialized (#5326)

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@26731 f0d54148-0727-0410-94bb-9a71ac55c965