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.

pkgs/os-specific/darwin/setfile/default.nix

@@ -20,7 +20,6 @@ stdenv.mkDerivation {
   meta = with stdenv.lib; {
     description = "Set attributes of files and directories";
     homepage    = "http://developer.apple.com/library/mac/#documentation/Darwin/Reference/ManPages/man1/setfile.1.html";
-    license     = licenses.unfree;
     maintainers = with maintainers; [ lovek323 ];
     platforms   = platforms.darwin;
 
@@ -32,4 +31,3 @@ stdenv.mkDerivation {
     '';
   };
 }
-