Remove some cases for turning off arf.

Flipping arf on and off too often is hurting some clips.
This change makes no difference for 50-75% of our test
clips but helps some by a big margin. (eg. std-hd crew
by 6% and one of the YT and YT-hd clips by 14%)

Average improvements for 2 pass, speed 2 (psnr,ssim)
are as follows:-

derf  0.165%, 0.210%
yt  1.210%, 1.464%
yt-hd 1.189%, 1.471%
std-hd 1.031%, 0.886%

Change-Id: I121fe66cfb4a62d384b23b484a7d648789641969