darwin.darwin-stubs: remove and replace with stubs

These are a different kind of stubs that do nothing except tell you to
check the documentation on how to use the new Darwin SDK instead.

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

@@ -1,18 +0,0 @@
-{ stdenvNoCC, fetchurl }:
-
-stdenvNoCC.mkDerivation {
-  pname = "darwin-stubs";
-  version = "10.12";
-
-  src = fetchurl {
-    url = "https://github.com/NixOS/darwin-stubs/releases/download/v20201216/10.12.tar.gz";
-    sha256 = "1fyd3xig7brkzlzp0ql7vyfj5sp8iy56kgp548mvicqdyw92adgm";
-  };
-
-  dontBuild = true;
-
-  installPhase = ''
-    mkdir $out
-    mv * $out
-  '';
-}

pkgs/top-level/darwin-packages.nix

@@ -84,6 +84,7 @@ makeScopeWithSplicing' {
     "architecture"
     "configd"
     "configdHeaders"
+    "darwin-stubs"
   ] (mkStub apple_sdk.version);
 in
 impure-cmds // appleSourcePackages // stubs // {
@@ -148,8 +149,6 @@ impure-cmds // appleSourcePackages // stubs // {
 
   DarwinTools = callPackage ../os-specific/darwin/DarwinTools { };
 
-  darwin-stubs = callPackage ../os-specific/darwin/darwin-stubs { };
-
   print-reexports = callPackage ../os-specific/darwin/print-reexports { };
 
   rewrite-tbd = callPackage ../os-specific/darwin/rewrite-tbd { };