vtkGhostCellsGenerator: input ghost peeling optim for polydata

When an input poly data has ghosts, there used to be a deep copy of the input into a temporary with its ghosts peeled off (calling RemoveGhosts). This commit gets rid of this temporary and directly maps the output cells and points with ghosts removed to the input cells and points.

Note: Since poly data and unstructured grids share a lot of code, the code also exists for unstructured grids. However, it has never been tested with input ghosts and cannot actually be run because ghosts are still removed prior to calling the ghost cells generator pipeline. This is because currently, the filter extracting the interfaces of the inputs doesn't handle ghost cells as this filter would need them to be handled. When vtkDataSetSurfaceFilter or vtkGeometryFilter can extract a surface in the same way vtkFeatureEdges does it (by simply skipping ghost cells in the input), this optimization can be added for unstructured grids (and some debugging performed, if needed).