Clear tasks when transaction is rollback

The tasks records are removed on rollback so there is no need to try to run
them.
(grafted from d2e0256f3478213d5c1ab375bc842c1222157692)

--HG--
branch : 6.0

trytond/trytond/protocols/dispatcher.py

@@ -189,6 +189,7 @@ def _dispatch(request, pool, *args, **kwargs):
             except backend.DatabaseOperationalError:
                 if count and not rpc.readonly:
                     transaction.rollback()
+                    transaction.tasks.clear()
                     continue
                 logger.error(log_message, *log_args, exc_info=True)
                 raise