Remove 3rd party lua plugin support

[rikatz]

What this PR does / why we need it:

For the sake of maintenance and security, we should stop allowing ingress-nginx to support 3rd party plugins and lua scripts.

We are aiming to have a much simpler implementation, and this PR removes this support

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[rikatz]

/priority important-longterm

[tao12345666333]

/lgtm
/hold

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rikatz, tao12345666333

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[rikatz]

/hold cancel

[jsoref]

I'm not sure what the versioning algorithm is for ingress-nginx, but as an end user, I'd kinda hope this results in at least a minor, if not a major version bump.

[rikatz]

@jsoref it will be on v1.12 (minor, as Kubernetes does), but not a patch release.

[callppatel]

does it mean, we will not able to use Lua Script in NGINX anymore?

[rikatz]

Not 3rd party, unless you modify the docker image and template