Revert "Add missing Security framework for macOS"

[cole-h]

Reverts DeterminateSystems/riff#63

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This doesn't work since it unconditionally adds the Security framework, which does not work on Linux:

error: Package ‘MacOS_SDK-10.12’ in /nix/store/8x70xcqlc9h9njwqa7hk73ansazxy40r-source/pkgs/os-specific/darwin/apple-sdk/default.nix:50 is not supported on ‘x86_64-linux’, refusing to evaluate.

       a) To temporarily allow packages that are unsupported for this system, you can use an environment variable
          for a single invocation of the nix tools.                                    

            $ export NIXPKGS_ALLOW_UNSUPPORTED_SYSTEM=1                                

        Note: For `nix shell`, `nix build`, `nix develop` or any other Nix 2.4+        
        (Flake) command, `--impure` must be passed in order to read this               
        environment variable.                                                          

       b) For `nixos-rebuild` you can set                                              
         { nixpkgs.config.allowUnsupportedSystem = true; }                             
       in configuration.nix to override this.                                          

       c) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add                                  
         { allowUnsupportedSystem = true; }                                                                             
       to ~/.config/nixpkgs/config.nix.                     

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Oops @ me for not testing this before merging.

We'll need to wait for https://github.com/DeterminateSystems/riff/pull/58 before we can fix this for ourselves.

[grahamc]

We'll need to re-implement this once https://linear.app/detsys/issue/DS-245/fsm-figure-out-how-to-deal-with-platform-specific-packages lands. Arg :)