libexpr: throw a more helpful eval-error if a builtin is not available due to a missing feature-flag

I found it somewhat confusing to have an error like

    error: attribute 'getFlake' missing

if the required experimental-feature (`flakes`) is not enabled. Instead,
I'd expect Nix to throw an error just like it's the case when using e.g. `nix
flake` without `flakes` being enabled.

With this change, the error looks like this:

    $ nix-instantiate -E 'builtins.getFlake "nixpkgs"'
    error: Cannot call 'builtins.getFlake' because experimental Nix feature 'flakes' is disabled. You can enable it via '--extra-experimental-features flakes'.

           at «string»:1:1:

                1| builtins.getFlake "nixpkgs"
                 | ^

I didn't use `settings.requireExperimentalFeature` here on purpose
because this doesn't contain a position. Also, it doesn't seem as if we
need to catch the error and check for the missing feature here since
this already happens at evaluation time.

src/libexpr/primops.hh

@@ -15,7 +15,6 @@ struct RegisterPrimOp
         std::vector<std::string> args;
         size_t arity = 0;
         const char * doc;
-        std::optional<std::string> requiredFeature;
         PrimOpFun fun;
     };
 
@@ -28,8 +27,7 @@ struct RegisterPrimOp
     RegisterPrimOp(
         std::string name,
         size_t arity,
-        PrimOpFun fun,
-        std::optional<std::string> requiredFeature = {});
+        PrimOpFun fun);
 
     RegisterPrimOp(Info && info);
 };