umami: drop dead umami-secrets ignoreDifferences stanza (3c)

Old out-of-band umami-secrets deleted (kubectl); the /data ignoreDifferences
stanza referencing it is now dead. Removed from both the registered app
manifest and the in-dir duplicate. umami-secrets-infisical is Owner-managed
by the operator, not a git-tracked Secret, so no ignore rule is needed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

umami/argocd-app.yaml

@@ -12,13 +12,6 @@ spec:
   destination:
     server: https://kubernetes.default.svc
     namespace: umami
-  ignoreDifferences:
-    - group: ""
-      kind: Secret
-      name: umami-secrets
-      namespace: umami
-      jsonPointers:
-        - /data
   syncPolicy:
     automated:
       prune: true