DerivationBuilderImpl::writeStructuredAttrs remove a rewrite

As much as I prefer rewriting the parsed rather than unparsed JSON for
elegance, this gets in the way of the separation of concerns that I am
trying to do.

As a practical matter, any rewriting that this did will also be done by
the second round of rewriting that remains below, so removing this code
should have no effect.

src/libstore/unix/build/derivation-builder.cc

@@ -1106,14 +1106,6 @@ void DerivationBuilderImpl::writeStructuredAttrs()
 {
     if (drv.structuredAttrs) {
         auto json = drv.structuredAttrs->prepareStructuredAttrs(store, drvOptions, inputPaths, drv.outputs);
-        nlohmann::json rewritten;
-        for (auto & [i, v] : json["outputs"].get<nlohmann::json::object_t>()) {
-            /* The placeholder must have a rewrite, so we use it to cover both the
-               cases where we know or don't know the output path ahead of time. */
-            rewritten[i] = rewriteStrings((std::string) v, inputRewrites);
-        }
-
-        json["outputs"] = rewritten;
 
         auto jsonSh = StructuredAttrs::writeShell(json);