[freeflow][cleanup] Delete all box-related models and tests

dumux/freeflow/CMakeLists.txt

dumux/freeflow/boundarylayermodel.hh

dumux/freeflow/masstransfermodel.hh

dumux/freeflow/stokes/CMakeLists.txt

dumux/freeflow/stokes/fluxvariables.hh

dumux/freeflow/stokes/indices.hh

dumux/freeflow/stokes/localjacobian.hh

dumux/freeflow/stokes/localresidual.hh

dumux/freeflow/stokes/model.hh

dumux/freeflow/stokes/newtoncontroller.hh

dumux/freeflow/stokes/problem.hh

dumux/freeflow/stokes/properties.hh

dumux/freeflow/stokes/propertydefaults.hh

dumux/freeflow/stokes/volumevariables.hh

dumux/freeflow/stokesnc/CMakeLists.txt

dumux/freeflow/stokesnc/fluxvariables.hh

dumux/freeflow/stokesnc/indices.hh

dumux/freeflow/stokesnc/localresidual.hh

dumux/freeflow/stokesnc/model.hh

dumux/freeflow/stokesnc/properties.hh