Remove faulty debug assertion

The use of the server state is not transactional across multiple uses of
the function. This means that any assertions on the target state can fail
if the monitor updates the state between target selection and the
assertion.

server/modules/routing/readwritesplit/rwsplit_route_stmt.cc

@@ -934,7 +934,6 @@ SRWBackend RWSplitSession::handle_slave_is_target(uint8_t cmd, uint32_t stmt_id)
     {
         mxb::atomic::add(&m_router->stats().n_slave, 1, mxb::atomic::RELAXED);
         m_server_stats[target->server()].read++;
-        mxb_assert(target->in_use() || target->can_connect());
     }
     else
     {