remove alertmanager configuration for legacy prometheus instances
What
Removes the alertmanager configuration from the legacy prometheus instances in GKE.
Why
Since we have migrated to Mimir this enables us to remove alert routing from the old prometheus infra. We will continue to collect data for a period but this will ensure all routed alerts are only originating from Mimir.
Related Issue: gitlab-com/gl-infra/scalability#3572 (closed)