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>

argocd/application-umami.yaml

@@ -9,13 +9,6 @@ spec:
   destination:
     namespace: umami
     server: https://kubernetes.default.svc
-  ignoreDifferences:
-  - group: ''
-    jsonPointers:
-    - /data
-    kind: Secret
-    name: umami-secrets
-    namespace: umami
   project: default
   source:
     path: umami