lint: remove unused items from resource, sync, platform, db

- Remove contentUploadList/Placeholder/Detail from resource/upload.go
- Remove toImageURLs, userURI from resource/communication.go
- Remove responseListUser from resource/user.go
- Remove contentSignin/getSignin from sync/signin.go (re-add contentSignin used elsewhere)
- Delete sync/service-request.go (entirely unused)
- Remove _rowWithID from platform/signal.go
- Remove unused tag field from db/tx.go Rows struct

resource/user.go

@@ -71,10 +71,6 @@ func (res *userR) response(u *platform.User) (*user, error) {
 type userR struct {
 	router *router
 }
-type responseListUser struct {
-	Users []*platform.User `json:"users"`
-}
-
 func (res *userR) ByIDGet(ctx context.Context, r *http.Request, user platform.User, query QueryParams) (*platform.User, *nhttp.ErrorWithStatus) {
 	vars := mux.Vars(r)
 	user_id_str := vars["id"]