- vtkPolygon::Triangulate returns local ids.
- vtkCell::Triangulate returns global ids.

It is confusing to have two functions with the same name, but
returning different things.

The previous commits introduced vtkCell::TriangulateLocalIds
that turns out to return the same things as vtkPolygon::Triangulate
So this commit deprecate vtkPolygon::Triangulate in favor of
vtkCell::TriangulateLocalIds.