feat(3p/nix): remove builtins.importNative

This is the shared object equivalent of builtins.exec, or a plugins
equivalent accessible from the Nix language.  Either way, since we
don't have builtins.exec or plugins any more, I think it makes sense
to remove this builtin.

This will also allow us to drop the
allow-unsafe-native-code-during-evaluation option, which formerly
controlled whether builtins.exec and builtins.importNative were
enabled.

Cc: Griffin Smith <grfn@gws.fyi>
Cc: Profpatsch <mail@profpatsch.de>
Change-Id: I8993a8a79d559c102647308a2684c089bbc06713
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1340
Reviewed-by: glittershark <grfn@gws.fyi>
Reviewed-by: Profpatsch <mail@profpatsch.de>
Tested-by: BuildkiteCI

third_party/nix/src/libexpr/primops.hh

@@ -14,9 +14,4 @@ struct RegisterPrimOp {
   RegisterPrimOp(const std::string& name, size_t arity, PrimOpFun fun);
 };
 
-/* These primops are disabled without enableNativeCode */
-/* Load a ValueInitializer from a DSO and return whatever it initializes */
-void prim_importNative(EvalState& state, const Pos& pos, Value** args,
-                       Value& v);
-
 }  // namespace nix