Remove seemingly redundant nix fix

Removing this seems to fix the nix build on my old mac. Hopefully this
fixes CI too.

I'd guess the fix now already exists somewhere upstream.

nix/haskell.nix

@@ -203,12 +203,6 @@ let
 
       # Build fixes for library dependencies
       {
-        # Make the /usr/bin/security tool available because it's
-        # needed at runtime by the x509-system Haskell package.
-        packages.x509-system.components.library.preBuild = lib.optionalString (stdenv.isDarwin) ''
-          substituteInPlace System/X509/MacOS.hs --replace security /usr/bin/security
-        '';
-
         # Packages we wish to ignore version bounds of.
         # This is similar to jailbreakCabal, however it
         # does not require any messing with cabal files.