fix(harbor): remove invalid secretName parameter from core config

The secretName parameter was being used as a literal secret name
'CORE_SECRET' which doesn't exist. This caused harbor-core pods to
fail mounting volumes.

The correct approach is to use existingSecret for credential keys,
and let Harbor manage its own internal secrets.

cluster/platform/harbor/values.yaml

@@ -131,7 +131,6 @@ core:
   # Core secret from ExternalSecret
   secret: ""
   existingSecret: "harbor-credentials"
-  secretName: "CORE_SECRET"
   # Enable HTTP/2 for better performance
   configureUserSettings: |-
     http2_push_preload on;