cleanup: remove container-engine flag from the delete config

[nXtCyberNet]

Changes

• remove the container-engine flag from cmd/cluster.go ,
• modified user template
• modified documation

/kind cleanup

Fixes #3979

Release Note

Docs

[linux-foundation-easycla]

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• ✅ login: nXtCyberNet / name: nXtCyberNet (4166079)

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[knative-prow]

Welcome @nXtCyberNet! It looks like this is your first PR to knative/func 🎉

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