(3.0) Remove existing "request payload" functionality

[cowtowncoder]

In Jackson 2.8, an API endpoint was added to attach "request payload" with parser. I think that API does not fit well with the design and needs to be rethought. As such, I'll remove it from 3.0.0-SNAPSHOT (master).
This removes one more aspect of mutability: if source is to be attached, it should be handled through builder-style mechanism (probably via ObjectReadContext somehow; that would allow passing it via ObjectReader).

But it definitely should not be done through setter in JsonParser for 3.0.