Drop macOS SDKs that follow "the old way"

Nixpkgs now has much more reasonable handling of macOS SDKs, and this old default is not generally needed  for builds anymore.

I think having these SDKs in the inputs is a bit of a nuisance now, since it means there are mixed framework versions in the closure.

See: https://discourse.nixos.org/t/on-the-future-of-darwin-sdks-or-how-you-can-stop-worrying-and-put-the-sdk-in-build-inputs/50574
See: NixOS/nixpkgs#346043

default.nix

@@ -1,5 +1,4 @@
 { cargo
-, darwin
 , fetchurl
 , jq
 , lib