MXS-1503: Remove false debug assertion

It is possible, and perfectly OK, for the new master to be in use at the
same time the old master is. This is the case if one of the slaves is
promoted as the master.

server/modules/routing/readwritesplit/rwsplit_route_stmt.cc

@@ -716,7 +716,6 @@ static void log_master_routing_failure(RWSplitSession *rses, bool found,
     {
         /** We found a master but it's not the same connection */
         ss_dassert(!rses->rses_config.master_reconnection);
-        ss_dassert(!curr_master->in_use());
         ss_dassert(old_master != curr_master);
         sprintf(errmsg, "Master server changed from '%s' to '%s'",
                 old_master->name(), curr_master->name());