[cleanup] Stop using header arithmetic.hh

The header ws once experimentally added to dune-fufem and is superseded by Dune::MatrixVector itself. Hence we should not use it here, but the analogues from Dune::matrixVector.

Disclaimer: This is not tested and I can't even find places where I use it. Nevertheless the change is straight forward.

mentioned in commit 3d6a332b

merged