chore(prometheus): drop dead grafana routes and cert

No grafana is deployed from this stack, so grafana-prod/grafana-local only ever served 503s. Live objects already deleted directly; this keeps git from resurrecting them on the next deploy.

prometheus-stack/k8s/certificates.yaml

@@ -1,18 +1,5 @@
 apiVersion: cert-manager.io/v1
 kind: Certificate
-metadata:
-  name: grafana-prod-tls
-  namespace: prometheus
-spec:
-  secretName: grafana-prod-tls
-  dnsNames:
-    - grafana.forust.xyz
-  issuerRef:
-    name: letsencrypt-prod
-    kind: ClusterIssuer
----
-apiVersion: cert-manager.io/v1
-kind: Certificate
 metadata:
   name: internal-wildcard-tls
   namespace: prometheus