removed potentials from dec2p2c because it is not used and access to the...

removed potentials from dec2p2c because it is not used and access to the vectro incredibly slow for large domains

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@5125 2fb0f335-1f38-0410-981e-8018bf24f1b0