Remove dependency on alert manager

[williewheeler]

Alert Manager is downstream in the metric processing pipeline, and AA shouldn't depend on it. If we want to provide some kind of simple default alerter then we should just create a simple component as part of AA itself. But I would suggest that we just leave alerting out altogether.

I know that it's a little weird that the project is called "Adaptive Alerting", in light of the above. :)