[!371] [cmake] Don't require Fortran when looking for arpack

Merge branch 'feature/dont-require-fortran-for-arpack' into 'master'

ref:core/dune-istl\> dune-common does no longer require / look for Fortran, so
the condition would be always false.

See merge request [core/dune-istl!371]

  [core/dune-istl!371]: gitlab.dune-project.org/core/dune-istl/merge_requests/371