thc-hydra: remove obsolete TARGET_OS_* workaround

Author: emilazy

pkgs/by-name/th/thc-hydra/package.nix

@@ -43,8 +43,6 @@ stdenv.mkDerivation rec {
         --replace-fail "-lcurses" "-lncurses"
     '';
 
-  env.NIX_CFLAGS_COMPILE = lib.optionalString stdenv.hostPlatform.isDarwin "-Wno-undef-prefix";
-
   nativeBuildInputs = lib.optionals withGUI [
     pkg-config
     makeWrapper