Remove dual prediction frame re-encoding loop.

I'm basically not convinced that the concept works at all, let alone
that this is the right place to do it. I think if we want something
like this at all, I should integrate it with the main encoding loop
and re-encode checks in onyx_if.c, and show that it has a significant
benefit (which right now, it doesn't; removing this re-encode check
actually increases all metrics by ~0.15%).

Change-Id: I1b597385dc17f468384a994484fb24813389411f