GridFunctionSpaceOrderingInterface is used nowhere?

GridFunctionSpaceOrderingInterface and its containing header dune/pdelab/ordering/orderinginterface.hh appear to be a leftover from the old ordering mechanism which was used previous to PDELab 2.0, since they are used nowhere inside of the module itself.

We might thus want to remove them (which would be a pitty because it is the only ordering related class/header which is well documented yet 😉).