alert-hub(staging): use prod preparedness api

Author: thenav56

applications/argocd/staging/applications/alert-hub-backend.yaml

@@ -36,8 +36,6 @@ spec:
             AZURE_STORAGE_STATIC_CONTAINER: alert-hub-staging-static-container
             AZURE_STORAGE_ACCOUNT_NAME: ifrctgo
             AZURE_STORAGE_MANAGED_IDENTITY: "true"
-            # Temporary
-            PREPAREMESSAGES_API_DOMAIN: https://api-preparemessages-stage.ifrc.org
           serviceAccount:
             annotations:
               azure.workload.identity/client-id: "99dd63fe-721e-4abb-b30d-e2b782d2893d"