Remove obsolete RequestHandler.async_callback method

[christarnowski]

This method is obsolete since Tornado 1.1, yet some of the code (e.g. in tornado.auth) still relies on it. The goal here would be to clean-up the codebase, including doc strings, and remove the method altogether.

[pjtait]

This change broke the "Long Polling" example in the "Introduction to Tornado" book (Chapter 5). Most unhelpful to those like me trying to get started with Tornado. Especially when there doesn't seem to be much help available to figure out how to convert existing code.