Remove wrong assertion from transaction::cleanup()

In case if SR transaction gets BF aborted before it is
certified, the replay will happen but if the replay fails
due to certification conflict, the transaction is not terminated
in provider replay and will keep its streaming status.

Removed the wrong assertion about streaming status from
transaction::cleanup().

Author: temeo

src/transaction.cpp

@@ -2084,7 +2084,6 @@ int wsrep::transaction::replay(wsrep::unique_lock<wsrep::mutex>& lock)
 void wsrep::transaction::cleanup()
 {
     debug_log_state("cleanup_enter");
-    assert(is_streaming() == false);
     assert(state() == s_committed || state() == s_aborted);
     id_ = wsrep::transaction_id::undefined();
     ws_handle_ = wsrep::ws_handle();