[Doxygen] Revert r11948, this hack is obsolete.

In r11948 I disabled the dot/GraphViz support because it
caused too big tarballs (>100 MiB).
The problem is now solved in dune-common …5598893bc702a
by setting INCLUDE_GRAPH to NO.
(reviewed by kissinger)


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@12307 2fb0f335-1f38-0410-981e-8018bf24f1b0