Disable zero bin mode boost.

As things stand the zero bin mode boost is hurting somewhat.
In part this seems to be because the boost applied as is
interferes with the rd mode selection loop.

Average gains (derf 0.072, yt 0.243, ythd 0.179 std-hd 0.212%)

Change-Id: Icaecea3908d9a7352370e49b8fa822f2c2c49dc1