docs: remove wrong comment on postgresql credentials secret (#882)

Author: xeniape

docs/modules/airflow/examples/getting_started/code/airflow-credentials.yaml

@@ -11,7 +11,6 @@ stringData:
   adminUser.email: airflow@airflow.com
   adminUser.password: airflow
 ---
-# Only needed when using celery workers (instead of Kubernetes executors)
 apiVersion: v1
 kind: Secret
 metadata: