greybird: remove license cc-by-nc-sa-30 (#46802)

Greybird is dual-licensed as GPLv2 or later and CC-BY-SA 3.0 or later.
The first is free, and the second is unfree in nixpkgs. Currently both
licenses are listed in the package derivation. And nix takes that it is
unfree. If one of the licenses in the list is unfree. nix consider
that the software is unfree. Remove the unfree one.

(cherry picked from commit 1944def)

pkgs/misc/themes/greybird/default.nix

@@ -19,7 +19,7 @@ stdenv.mkDerivation rec {
   meta = {
     description = "Grey and blue theme (Gtk, Xfce, Emerald, Metacity, Mutter, Unity)";
     homepage = https://github.com/shimmerproject/Greybird;
-    license = with stdenv.lib.licenses; [ gpl2Plus cc-by-nc-sa-30 ];
+    license = with stdenv.lib.licenses; [ gpl2Plus ]; # or alternatively: cc-by-nc-sa-30
     platforms = stdenv.lib.platforms.linux;
     maintainers = [ stdenv.lib.maintainers.romildo ];
   };