treewide: get rid of platforms.allBut

Negative reasoning like `allBut` is a bad idea with an open world of
platforms. Concretely, if we add a new, quite different sort of
platform, existing packages with `allBut` will claim they work on it
even though they probably won't.

pkgs/development/libraries/cln/default.nix

@@ -15,6 +15,6 @@ stdenv.mkDerivation rec {
     description = "C/C++ library for numbers, a part of GiNaC";
     homepage = http://www.ginac.de/CLN/;
     maintainers = [ ];
-    platforms = with stdenv.lib.platforms; allBut cygwin;
+    platforms = stdenv.lib.platforms.unix; # Once had cygwin problems
   };
 }