Remove unused code in the grid module

Description

The removed code created an union between the derivative stencil and a cube stencil of order two with the comment that this is used in multigrid. However, this is not the case anymore - multigrid uses the same stencil as the general grid. Thus, gr%stencil is not needed anymore as also stencil_union.

Checklist

• I have checked that my code follows the Octopus coding standards