Remove ObjectMapper duplicate

[juancki]

I understand that this project is stale and this might be talking to a void, etc...

However, apparently the class JsonBodyHandler.java is referenced in Twilio's article about making HTTP requests in Java.

The class defines a static ObjectMapper in here that is never used.

And then it creates a new ObjectMapper each time the function is called in here. This is totally against Jackson's recommendations on using ObjectMapper.