1p2c box model: fix compilation

it does not make sense to provide both, a phaseDensity() and a molarDensity()
function in the fluidsystems because they are connected simply by dividing the
mass density by the mean molar mass.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@5167 2fb0f335-1f38-0410-981e-8018bf24f1b0