libvpx

Remove brightness weighting in two pass.

This code dates from the ancient past and
applied an error score weighting based on pixel
brightness. This not seem to be providing any
benefit metrics wise and could be making some
visual issues in dark frames worse.

The field is left in place in the FIRSTPASS_STATS data
structure in this patch, pending changes to unit tests that
use a pre-defined first pass file.

Change-Id: Id50f04205230234858e7548ce523f11acaf3567d