intrapred,smooth_predictor: rm unnecessary saturation
intrapred,smooth_predictor: rm unnecessary saturation

with the current max weight (255) the accumulation and rounding value
(128/256) cannot exceed the range enforced by the descale (8/9).

BUG=aomedia:1538

Change-Id: Iab258841f050d1bb8821532303ad2406e2604616

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom