vtkCountFaces & vtkCountVertices: Optimize implicit array implementation

Follow-up to !10983 (merged) optimizing the Count Faces and Vertices filters using implicit arrays. Both filters used to call the GetCell method on every cell, which is much slower than using vtkDataSet functions.

Implicit array implementations are now faster than non-implicit array ones for structured data while keeping a lower memory consumption, so they are now the default and only option (no deprecation because UseImplicitArray was introduced very recently)