libvpx

Remove rate correction factor.

Removed an adaptive rate correction factor that was having
a negative impact on quality in many clips. This factor
was influencing the Q range available to each frame
independently of the bits allocated to each.

Average results with DISABLE_RC_LONG_TERM_MEM.

derf +0.199, -0.059.
yt +3.957, +3.798
std hd +1.577, +2.140
yt hd +4.127, +4.513

Average results without  DISABLE_RC_LONG_TERM_MEM

derf -0.628, -0.665
yt +3.432, +3.015
std hd -0.105, +0.153
yt hd +3.432, +3.015

Change-Id: I45bab6b606f49a442e7b27a6d631f3ffd843bbce