darwin.apple-sdk: drop appleSdkVersion

In fact no one is using appleSdkVersion, and stdenv is decoupled with sdk.

pkgs/stdenv/darwin/default.nix

@@ -1,7 +1,5 @@
 { lib
 , localSystem, crossSystem, config, overlays, crossOverlays ? []
-# The version of darwin.apple_sdk used for sources provided by apple.
-, appleSdkVersion ? "10.12"
 # Minimum required macOS version, used both for compatibility as well as reproducability.
 , macosVersionMin ? "10.12"
 # Allow passing in bootstrap files directly so we can test the stdenv bootstrap process when changing the bootstrap tools
@@ -150,7 +148,7 @@ in rec {
         __extraImpureHostDeps = commonImpureHostDeps;
 
         extraAttrs = {
-          inherit macosVersionMin appleSdkVersion platform;
+          inherit macosVersionMin platform;
         };
         overrides  = self: super: (overrides self super) // {
           inherit ccNoLibcxx;
@@ -524,7 +522,7 @@ in rec {
     extraAttrs = {
       libc = pkgs.darwin.Libsystem;
       shellPackage = pkgs.bash;
-      inherit macosVersionMin appleSdkVersion platform bootstrapTools;
+      inherit macosVersionMin platform bootstrapTools;
     };
 
     allowedRequisites = (with pkgs; [