Removed false debug assertion in readwritesplit

The readwritesplit assumed that the execution of a session command would
always succeed. This is not the case when a write to the backend server
fails and it is not something that shouldn't happen.

server/modules/routing/readwritesplit/readwritesplit.c

@@ -2906,7 +2906,7 @@ static void clientReply (
                      bref->bref_backend->backend_server->port);
 
                 succp = execute_sescmd_in_backend(bref);
-		ss_dassert(succp);
+
 		if(!succp)
 		{
 		    MXS_INFO("Backend %s:%d failed to execute session command.",