darwin.usr-include: remove

Originally introduced in 8610a344 ('gcc: use special native system
headers for darwin') as a hack, not used anymore.

pkgs/os-specific/darwin/usr-include/default.nix

@@ -1,23 +0,0 @@
-{lib, stdenv, darwin}:
-
-/*
- * This is needed to build GCC on Darwin.
- *
- * These are the collection of headers that would normally be available under
- * /usr/include in macOS machines with command line tools installed. They need
- * to be in one folder for gcc to use them correctly.
- */
-
-stdenv.mkDerivation {
-  name = "darwin-usr-include";
-  buildInputs = [ darwin.CF stdenv.libc ];
-  buildCommand = ''
-    mkdir -p $out
-    cd $out
-    ln -sf ${stdenv.libc}/include/* .
-    mkdir CoreFoundation
-    ln -sf ${darwin.CF}/Library/Frameworks/CoreFoundation.framework/Headers/* CoreFoundation
-  '';
-
-  meta.platforms = lib.platforms.darwin;
-}

pkgs/top-level/darwin-packages.nix

@@ -154,8 +154,6 @@ impure-cmds // appleSourcePackages // chooseLibs // {
 
   trash = callPackage ../os-specific/darwin/trash { };
 
-  usr-include = callPackage ../os-specific/darwin/usr-include { };
-
   xattr = pkgs.python3Packages.callPackage ../os-specific/darwin/xattr { };
 
   inherit (pkgs.callPackages ../os-specific/darwin/xcode { })