Drop obsolete graphviz dependency

Dependencies

• [2] L7RW467Z Add a test for darcs inputs.
• [3] T4LLYESZ * Nix expression for building Hydra.
• [4] LOHWNXEJ Cleanup

Change contents

• replacement in release.nix at line 162
  [3.416]→[2.78:179](∅→∅)
  − gzip bzip2 lzma gnutar unzip git gitAndTools.topGit mercurial darcs gnused graphviz bazaar
  [3.416]

  [3.511]
  + gzip bzip2 lzma gnutar unzip git gitAndTools.topGit mercurial darcs gnused bazaar