SSS: don't set txn_id

The field got dropped on the transition from MSC3575 to MSC4186, and
including it anyway confuses matrix-rust-sdk into trying to use the
(also removed) "sticky" parameters.

src/api/client_server/sync/msc4186.rs

@@ -615,7 +615,7 @@ pub(crate) async fn sync_events_v5_route(
     }
 
     Ok(Ra(sync_events::v5::Response {
-        txn_id: body.txn_id.clone(),
+        txn_id: None,
         pos: next_batch.to_string(),
         lists,
         rooms,