remove decay ratio in AssetIssueContract

Author: nanfengpo

core/Contract.proto

@@ -89,7 +89,6 @@ message AssetIssueContract {
   int32 num = 8;
   int64 start_time = 9;
   int64 end_time = 10;
-  int32 decay_ratio = 15;
   int32 vote_score = 16;
   bytes description = 20;
   bytes url = 21;