Remove work-around for bad NULL definition


git-svn-id: https://gnunet.org/svn/gnunet@34767 140774ce-b5e7-0310-ab8b-a85725594a96
Rename DEFAULT_QUALITY to MLP_DEFAULT_QUALITY to avoid conflicting with W32 GDI

git-svn-id: https://gnunet.org/svn/gnunet@34768 140774ce-b5e7-0310-ab8b-a85725594a96