Fix a warning

g++, with `-Wall -Wextra`:

    .../include/CGAL/Mesh_3/polylines_to_protect.h:50:28: warning: unused parameter 'k' [-Wunused-parameter]

Mesh_3/include/CGAL/Mesh_3/polylines_to_protect.h

@@ -47,7 +47,7 @@ void snap_graph_vertices(Graph& graph,
                          const double vx, const double vy, const double vz,
                          PolylineInputIterator existing_polylines_begin,
                          PolylineInputIterator existing_polylines_end,
-                         K k)
+                         K)
 {
   const double dist_bound = (std::min)(vx,
                                        (std::min)(vy, vz)) / 256;