Helm: Change Ruler to not use alertmanager headless service.

[stevesg]

The jsonnet deployment configures the Ruler to use the cluster IP alertmanager service, whereas the Helm deployment uses the headless service. This change updates the Helm chart to match the jsonnet.

The reason we do not need to use a headless service for Mimir Alertmanager is because it has a distributor built in, so it's only necessary to send alerts to a single Alertmanager replica, and they will be then distributed to the correct three replicas for the tenant.

[dimitarvdimitrov]

@stevesg do you still want to merge this?

[stevesg]

Yep, I'll clean it up shortly. Just doing some testing internally before merging.