Commented unused variable

```
/home/msalinas/Documents/standalone-installations/cifParsers/libcifpp/src/pdb/cif2pdb.cpp: In function ‘std::tuple<int, int> cif::pdb::WriteCoordinatesForModel(std::ostream&, const cif::datablock&, const std::map<std::__cxx11::basic_string<char>, std::tuple<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, int, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >&, std::set<std::__cxx11::basic_string<char> >&, int)’:
/home/msalinas/Documents/standalone-installations/cifParsers/libcifpp/src/pdb/cif2pdb.cpp:3362:15: warning: unused variable ‘pdbx_nonpoly_scheme’ [-Wunused-variable]
 3362 |         auto &pdbx_nonpoly_scheme = db["pdbx_nonpoly_scheme"];
      |         
```
This warning appeared while compiling the library. The use of that variable below has been commented, so I think it's appropiate to do the same thing with the unused variable.

src/pdb/cif2pdb.cpp

@@ -3359,7 +3359,7 @@ std::tuple<int, int> WriteCoordinatesForModel(std::ostream &pdbFile, const datab
 	auto &atom_site_anisotrop = db["atom_site_anisotrop"];
 	auto &atom_site_anisotrop = db["atom_site_anisotrop"];
 	auto &entity = db["entity"];
 	auto &entity = db["entity"];
 	auto &pdbx_poly_seq_scheme = db["pdbx_poly_seq_scheme"];
 	auto &pdbx_poly_seq_scheme = db["pdbx_poly_seq_scheme"];
-	auto &pdbx_nonpoly_scheme = db["pdbx_nonpoly_scheme"];
+	//auto &pdbx_nonpoly_scheme = db["pdbx_nonpoly_scheme"];
 	auto &pdbx_branch_scheme = db["pdbx_branch_scheme"];
 	auto &pdbx_branch_scheme = db["pdbx_branch_scheme"];
 
 
 	int serial = 1;
 	int serial = 1;