A small rd fix for av1_merge_rd_stats

zero_rate should not be added.

Change-Id: I372b82f69915809869cbc352c2db44c963c31cfa

av1/encoder/rd.h

@@ -387,7 +387,6 @@ static INLINE void av1_merge_rd_stats(RD_STATS *rd_stats_dst,
   rd_stats_dst->rate += rd_stats_src->rate;
   rd_stats_dst->dist += rd_stats_src->dist;
   rd_stats_dst->sse += rd_stats_src->sse;
-  rd_stats_dst->zero_rate += rd_stats_src->zero_rate;
   rd_stats_dst->skip &= rd_stats_src->skip;
   rd_stats_dst->invalid_rate &= rd_stats_src->invalid_rate;
 #if CONFIG_RD_DEBUG