Remove api_rate_limit backend from haproxy 500 error alert.

The api_rate_limit backend can spike sometimes, and in the course of doing it's job, it triggers a wider alert that gives the impression that there is a major problem.

We should not alert on the api_rate_limit backend, at least not for a very short burst of activity. Instead, we should consider a more specific alert if the rate limit backend is seeing activity for a longer period of time, as this could indicate abuse or a problem.