Remove static alertmanager config

assets/alertmanager/alertmanager.yaml

-global:
-  resolve_timeout: 5m
-route:
-  group_by: ['job']
-  group_wait: 30s
-  group_interval: 5m
-  repeat_interval: 12h
-  receiver: 'null'
-  routes:
-  - match:
-      alertname: DeadMansSwitch
-    receiver: 'null'
-receivers:
-- name: 'null'