chore(3p/nix/libexpr): Remove dead code from attribute sets

These bits are no longer required with the hashmap-backed
implementation of attribute sets.

Change-Id: I8b936d8d438a00bad4ccf8e0b4dd719c559ce8c2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/912
Tested-by: BuildkiteCI
Reviewed-by: edef <edef@edef.eu>

third_party/nix/src/libexpr/eval.cc

@@ -1865,7 +1865,7 @@ size_t valueSize(Value& v) {
       case tAttrs:
         if (seen.find(v.attrs) == seen.end()) {
           seen.insert(v.attrs);
-          sz += sizeof(Bindings) + sizeof(Attr) * v.attrs->capacity();
+          sz += sizeof(Bindings);
           for (auto& i : *v.attrs) {
             sz += doValue(*i.second.value);
           }