Deprecate and remove some nonstandard uses of RTCPeerConnection legacy methods.

Deprecate and remove some nonstandard uses of RTCPeerConnection legacy methods. (removed)

The latest version of the WebRTC spec introduces new promise-based methods for the RTCPeerConnection interface while keeping support for legacy callback-based methods. Some nonstandard uses of the legacy methods prevent the implementation of the new promise-based methods due to overloading conflicts and they are now deprecated. More specifically, calling createOffer() and createAnswer() without providing a failure callback (mandatory in the spec, but optional in Chromium) is now deprecated.