Draft: Remove VirtualiozedGF of dune-vtk from dune-grid

The inclusion of code from dune-vtk and the implicity conversion from one grid function type-erasure class into another wrapper cases issues ub downstream code that wants to implement proper grid-functions based on the dune-functions defintion. The VirtualizedGF implicit conversion then causes ambiguities.

Note that this MR currently leads to non-working code in dune-vtk where we also couldn't implement the grid function in the python side due to the inclusion here in dune-grid.