[sync] override amz storage class, None to delete (#3639)

* override amz storage class, None to delete
https://github.com/seaweedfs/seaweedfs/issues/3636

* use empty string to delete

* without nil check

weed/command/filer_sync_jobs.go

@@ -54,7 +54,7 @@ func (t *MetadataProcessor) AddSyncJob(resp *filer_pb.SubscribeMetadataResponse)
 
 		// if is the oldest job, write down the watermark
 		isOldest := true
-		for t, _ := range t.activeJobs {
+		for t := range t.activeJobs {
 			if resp.TsNs > t {
 				isOldest = false
 				break