implicit 2pdfm: The TwoPDFMModel has always done the same as the TwoPModel....

implicit 2pdfm: The TwoPDFMModel has always done the same as the TwoPModel. The obsolete reimplementation of addOutputVtkFields has been removed. Reviewed by Christoph.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@10720 2fb0f335-1f38-0410-981e-8018bf24f1b0