os-specific/darwin/setfile: drop the "unfree" license

Setfile is included by other derivation, which in turns makes them unfree, too.
This causes plenty of evaluation errors on Hydra, i.e.:

  at `haskellPackages_ghc763_profiling.wx.x86_64-darwin' [nixpkgs = ..., officialRelease = false]:
  user-thrown exception: package ‘setfile’ has an unfree license, refusing to evaluate

Now, it's true that "setfile" is unfree, but this doesn't affect us: our
derivation doesn't include the actual binary -- it just contains a symlink to
"/usr/bin/SetFile". Arguably, our setfile derivation is free and we can
re-distribute it.