Remove VTK_GOURAUD interpolation mode
The VTK_GOURAUD interpolation mode is, in fact, a Phong interpolation mode.
There is also a VTK_PHONG interpolation mode, and the same shading mode is used whether we are in VTK_GOURAUD or VTK_PHONG mode.
We should :
- Remove the VTK_GOURAUD interpolation mode
- Keep the method SetInterpolationModeToGouraud and link it to SetInterpolationModeToPhong for compatibility