Remove references of vendor configurations in GatewayTLSConfig

[nak3]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

The references of vendor configurations in GatewayTLSConfig confuses the users.

Which issue(s) this PR fixes:

Fixes #759

[k8s-ci-robot]

Hi @nak3. Thanks for your PR.

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[nak3]

As v1alpha1 is frozen, I didn't change v1alpha1 file and so it did not update the doc. Please let me know if I should change it.

[robscott]

Thanks @nak3!

/ok-to-test

[robscott]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: nak3, robscott

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