[stokes]

- added DUNE_THROW in case some wants to use stokes in 3D
- removed unnecessary and unused code accidentily committed in r12468

reviewed by timok


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@15389 2fb0f335-1f38-0410-981e-8018bf24f1b0