parmetis: set license to unfree

generic unfree license fits best

pkgs/development/libraries/science/math/parmetis/default.nix

···
     description = "ParMETIS is an MPI-based parallel library that implements a variety of algorithms for partitioning unstructured graphs, meshes, and for computing fill-reducing orderings of sparse matrices";
     homepage = http://glaros.dtc.umn.edu/gkhome/metis/parmetis/overview;
     platforms = platforms.all;
-    license = licenses.cc-by-nc-sa-20;
+    license = licenses.unfree;
     maintainers = [ maintainers.costrouc ];
   };
 }