[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and was later itself superseded by dune-matrix-vector.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.