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