Revert r13462, it broke compatibility with Dune 2.3.

As we use still ints for mapping face indices, there
is more work involved.
(thanks to bernd for the heads-up)


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@13466 2fb0f335-1f38-0410-981e-8018bf24f1b0