refactor(tvix/libexpr): Remove Bindings::SortedByKeys()

Since we don't have a Bindings implementation with unstable order this
function is not required, as its callers can just iterate over the
attributes instead.

Change-Id: I01b35277b5a2dde69d684bc881dbd7c0701bcbb3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2291
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>

third_party/nix/src/libexpr/attr-set.hh

@@ -59,13 +59,6 @@ class Bindings {
   iterator end();
   const_iterator cend() const;
 
-  // Returns the elements of the attribute set as a vector, sorted
-  // lexicographically by keys.
-  //
-  // This is used primarily for builtins that have guaranteed
-  // ordering, such as `attrNames` or `attrValues`.
-  std::vector<const Attr*> SortedByKeys();
-
   // oh no
   friend class EvalState;