[cleanup] Remove incomplete Dune::BlockVectorWindow.
Dune::BlockVectorWindow
is declared, but never defined. Also get rid of
BlockVector
assignment from Dune::BlockVectorWindow
which can't possibly
have worked, since Dune::BlockVectorWindow
is incomplete, and there are no
issues or MRs in istl complaining about this.
Note that there is a similar but unrelated (except possibly historically)
Dune::Imp::BlockVectorWindow
.