Do not generate callback-style methods in ES6 mode

Currently, when generating a javascript API, both promise-style functions and callback-style functions are generated. An application will likely be using only one of them and when the app is using ES6, it will be Promises. Do not generate callback-style methods in ES6 mode, as it approximately doubles the bundle size.

The actual changes in the generator are rather simple, we are willing to work on this if we get some feedback that other people think this is a good idea.