feat: use batch download api for create multitasks (#978)

pkg/rest/model/task.go

@@ -7,8 +7,3 @@ type CreateTask struct {
 	Req *base.Request `json:"req"`
 	Opt *base.Options `json:"opt"`
 }
-
-type CreateTaskBatch struct {
-	Reqs []*base.Request `json:"reqs"`
-	Opt  *base.Options   `json:"opt"`
-}