prepare_proxy: do not redirect inbound traffic to Envoy

[rlenglet]

Disable redirecting all pod inbound traffic to Istio's Envoy sidecar proxy. Traffic is already filtered by Cilium when used in conjunction with Istio.

[istio-testing]

@rlenglet: Adding do-not-merge/release-note-label-needed because the release note process has not been followed.

One of the following labels is required "release-note", "release-note-action-required", or "release-note-none". Please see: https://github.com/kubernetes/community/blob/master/contributors/devel/pull-requests.md#write-release-notes-if-needed. Instructions for interacting with me using PR comments are available [here](https://github.com/kubernetes/community/blob/master/contributors/devel/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.

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[istio-testing]

Hi @rlenglet. Thanks for your PR.

I'm waiting for a istio member to verify that this patch is reasonable to test. If it is, they should reply with /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.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available [here](https://github.com/kubernetes/community/blob/master/contributors/devel/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository. I understand the commands that are listed [here](https://github.com/kubernetes/test-infra/blob/master/commands.md).