build(nix): remove unused dependencies

darwin.apple_sdk.frameworks is deprecated and does nothing according to
<https://nixos.org/manual/nixpkgs/stable/#sec-darwin-legacy-frameworks>

libiconv dependency also does not seem to be needed for `libdeltachat`,
I have checked that `nix build .#libdeltachat` still succeeds on macOS.

flake.nix

@@ -98,9 +98,6 @@
             nativeBuildInputs = [
               pkgs.perl # Needed to build vendored OpenSSL.
             ];
-            buildInputs = pkgs.lib.optionals isDarwin [
-              pkgs.darwin.apple_sdk.frameworks.SystemConfiguration
-            ];
             auditable = false; # Avoid cargo-auditable failures.
             doCheck = false; # Disable test as it requires network access.
           };
@@ -483,12 +480,6 @@
                   pkgs.rustPlatform.cargoSetupHook
                   pkgs.cargo
                 ];
-                buildInputs = pkgs.lib.optionals isDarwin [
-                  pkgs.darwin.apple_sdk.frameworks.CoreFoundation
-                  pkgs.darwin.apple_sdk.frameworks.Security
-                  pkgs.darwin.apple_sdk.frameworks.SystemConfiguration
-                  pkgs.libiconv
-                ];
 
                 postInstall = ''
                   substituteInPlace $out/include/deltachat.h \