Remove mentioning insecure server (which is not supported anymore) from API server docs

[nilebox]

What this PR does / why we need it:
Remove mentioning insecure serving from the docs, since only secure serving is supported now.

[k8s-ci-robot]

Hi @nilebox. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with @k8s-bot ok to test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[k8s-reviewable]

This change is 

[nilebox]

/assign @deads2k

[cblecker]

@k8s-bot ok to test

[deads2k]

/lgtm
/release-note-none

[k8s-github-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: deads2k, nilebox

Needs approval from an approver in each of these OWNERS Files:

• staging/src/k8s.io/apiserver/pkg/server/OWNERS [deads2k]

You can indicate your approval by writing /approve in a comment
You can cancel your approval by writing /approve cancel in a comment

[k8s-github-robot]

Automatic merge from submit-queue