Don't export the codimension as 'codim'

That export is redundant: the class already exports the codimension
as 'codimension'.  Besides, the 'codim' is apparently not used
anywhere in dune-grid.  The entire dune-grid test suite still builds
and runs for me.

Removal of the 'codim' field has been decided in
http://users.dune-project.org/doodles/16