removed calculation of unused velocityDivergence (commented) and
massFractionGradient (moleFractionGradient is used for the diffusion);
unfortunately, deprecation was not possible in this case

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@8067 2fb0f335-1f38-0410-981e-8018bf24f1b0