[box] simplify calculation of the boundary face normal

For ages, the method centerUnitOuterNormal exists which does not require
a local coordinate on the intersection. Therefore, we can also get rid
of the assumption.

Reviewed by gruenich.



git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@14957 2fb0f335-1f38-0410-981e-8018bf24f1b0