Remove alpha environment variable because feature is in beta

[ardaguclu]

Since kubernetes/enhancements#3895 has been promoted to beta, environment
variable representing alpha feature enablement has become obsolete. This PR removes this from website.

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[tengqm]

Should this target main (1.28) or dev-1.29 branch?

[ardaguclu]

Should this target main (1.28) or dev-1.29 branch?

ah, sorry. This should target to dev-1.29 branch. I'll fix it.

[ardaguclu]

Closing this in favor of #42907;

/close

[k8s-ci-robot]

@ardaguclu: Closed this PR.

In response to this:

Closing this in favor of #42907;

/close

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