Bug#884444: unnecessary libgraphviz-dev dependency (and graphviz suggests?)

[Matthias Klose]

Package: src:imagemagick
Version: 8:6.9.7.4+dfsg-16

imagemagick's b-d's have a commented out

# libgraphviz-dev, incompatible license against fftw

but libmagickcore-6.q16-dev still depends on libgraphviz-dev.  That dependency
should be removed.