[cleanup] Remove experimental header arithmetic.hh

This header was added a long time ago to replace staticmatrixtools.hh and was later itself superseded by dune-matrix-vector. Furthermore there was also a copy of this header in dune-solvers which was deprecated since 8 years. The header is not used anywhere and is thus removed. Since it was still documented as experimental, removal does not require a deprecation here.

enabled an automatic merge when all merge checks for 915b02ea pass

mentioned in commit 640783c7

merged