[RLlib] Remove framework_iterator from codebase.

[sven1977]

Remove framework_iterator from codebase.

In preparation of eventually retiring tf support. Simplifies our bloated test suite a bit.

Why are these changes needed?

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[simonsays1980]

LGTM. I smell the rise of torch :) Thanks for this ton of work that must have gone into finding and removing framework iterators!

[RocketRider]

It would be great if you could communicate your Roadmap, like removing support of tf. Not just reading it in some chances.