[Alerting] Allow disabling the Internal/Built-In AlertManager

[diranged]

Why is this needed:

We operate a central AlertManager service that lives outside of Grafana, but we want our engineers to use Grafana to see and interface with that service. I do not want them to be confused by having to pick between "Grafana" and "Alertmanager" in the "Choose Alertmanager" picker.. I want to only have one Alertmanager.

Please allow us to disable the internal alertmanager and finish this code: https://github.com/grafana/grafana/blob/v11.0.0/pkg/services/ngalert/ngalert.go#L173-L179

What would you like to be added:

The ability to enforce Grafana to ONLY use our remote AlertManager and not show any others.

Related PR

#81513 is related as well.