UnkeyedValidPathInfo::fromJSON Remove support for older version

It turns out this code path is only used for unit tests (to ensure our
JSON formats are possible to parse by other code, elsewhere). No
user-facing functionality consumes this format.

Therefore, let's drop the old version parsing support.

src/libutil/include/nix/util/json-utils.hh

@@ -114,4 +114,13 @@ struct adl_serializer<std::optional<T>>
     }
 };
 
+template<typename T>
+static inline std::optional<T> ptrToOwned(const json * ptr)
+{
+    if (ptr)
+        return std::optional{*ptr};
+    else
+        return std::nullopt;
+}
+
 } // namespace nlohmann