Remove mention of CeleryKubernetesExecutor from KubernetesExecutor doc

[alant94]

Removed mention of CeleryKubernetesExecutor as it is no longer supported, see here

[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contributors' Guide
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[shahar1]

Thanks for the cleanup, welcome to Apache Airflow!

[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.