decoupled tutorial: streamline the code a bit

we do not need to set any default arguments for the type tag in the
problem definition. the type tag which is actually used is chosen in
the main function.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@4805 2fb0f335-1f38-0410-981e-8018bf24f1b0