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/compilers/mono/generic.nix

@@ -88,7 +88,7 @@ stdenv.mkDerivation rec {
   meta = {
     homepage = http://mono-project.com/;
     description = "Cross platform, open source .NET development framework";
-    platforms = with stdenv.lib.platforms; allBut [ "aarch64-linux" ];
+    platforms = stdenv.lib.platforms.x86;
     maintainers = with stdenv.lib.maintainers; [ viric thoughtpolice obadz vrthra ];
     license = stdenv.lib.licenses.free; # Combination of LGPL/X11/GPL ?
   };