Remove the deprecation warning for a few RNode helpers

[mengqiy]

The Pipe method is not as intuitive as these helpers. These helpers
are convenient and useful. Deprecating them before we have better
alternatives is premature.

[k8s-ci-robot]

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[mengqiy]

/uncc mortent
/cc @natasha41575 @KnVerey

[natasha41575]

What do you envision for the future of these helpers and pipes? IIRC the reason for deprecating them was due to redundancy, do we eventually plan to remove one or the other or would we keep them both?

[mengqiy]

We want to deprecate and remove these 3 methods eventually. We will keep Pipe.
I imagine we can start to deprecate these 3 methods again after we merge #4319.
With #4319, we will provide only one function for fn developers to get a field: http://104.197.156.154/pkg/sigs.k8s.io/kustomize/kyaml/fn/framework/#KubeObject.Get

[natasha41575]

/lgtm