Clean vtkMRMLSpatialObjectNode from subsampling ratio

This subsampling ratio isn't used anywhere so there is no point in having
it around. Also make sure the polydata displayed is indeed the filtered
(i.e. cleaned) polydata.
Merge pull request #363 from vovythevov/CleanUnusedSubSampleRatio

Clean vtkMRMLSpatialObjectNode from subsampling ratio