feat(3p/nix): remove allow-unsafe-native-code-during-evaluation

This formerly controlled access to builtins.exec and
builtins.importNative, but both of those are gone now, so there's no
need for this option any more.

Change-Id: I6850cbd6be264fbfb1b209a60026dadbd0ba1232
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1341
Reviewed-by: glittershark <grfn@gws.fyi>
Tested-by: BuildkiteCI

third_party/nix/src/libexpr/eval.hh

@@ -331,11 +331,6 @@ struct InvalidPathError : EvalError {
 };
 
 struct EvalSettings : Config {
-  Setting<bool> enableNativeCode{this, false,
-                                 "allow-unsafe-native-code-during-evaluation",
-                                 "Whether builtin functions that allow "
-                                 "executing native code should be enabled."};
-
   Setting<bool> restrictEval{
       this, false, "restrict-eval",
       "Whether to restrict file system access to paths in $NIX_PATH, "