2p box model: delete unnecessary file 2pfluidstate.hh

bernd changed the 2p model to the immiscible fluid state. the other
box models will get a similar treatment in the future...

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@7051 2fb0f335-1f38-0410-981e-8018bf24f1b0