remove postBuild substitution from flux secrets configuration

Author: Mafyuh

kubernetes/cluster/production/flux-system/secrets.yaml

@@ -14,9 +14,3 @@ spec:
     secretRef:
       name: sops-age
   prune: true
-  postBuild:
-    substituteFrom:
-      - kind: ConfigMap
-        name: cluster-config
-      - kind: Secret
-        name: cluster-secrets