libvpx

Removes active_worst_quality feedback

Removes certain cases of feedback of active_worst_quality,
and removes it from the RATE_CONTROL structure. Now active
worst quality is expected to be computed locally in the
q picking function during the encode.
Making temporal filter strength depend on avg_frame_qindex
rather than on active_worst_quality actually improves
performance esp. for yt.
derf: +0.038%
yt: +0.359%

Change-Id: I1fe5a343034b55af9322289165321f00ac0827b1