AK: Remove unused JSON string APIs

Author: https://github.com/trflynn89
Commit: ee70f1327a
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/3631
Reviewed-by: https://github.com/awesomekling ✅

AK/JsonArray.h

@@ -78,8 +78,6 @@ public:
     template<typename Builder>
     void serialize(Builder&) const;
 
-    [[nodiscard]] ByteString to_byte_string() const { return serialized<StringBuilder>(); }
-
     template<typename Callback>
     void for_each(Callback callback)
     {