Tooling: Removes Istio

[danehans]

What type of PR is this?
/kind cleanup

What this PR does / why we need it:
Removes Istio-specific tooling from repo.

Which issue(s) this PR fixes:

Fixes #1505

Does this PR introduce a user-facing change?:

NONE

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: danehans

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[danehans]

@kfswain @robscott as I was removing Istio-specific tooling, I noticed GKE-specific content in hack/verify-manifests.sh and a few other places. When you have time, please create a similar tracker issue or a PR to do the same clean-up.

[k8s-ci-robot]

@danehans: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-gateway-api-inference-extension-verify-main	ccbcef9	link	true	/test pull-gateway-api-inference-extension-verify-main
pull-gateway-api-inference-extension-test-unit-main	ccbcef9	link	true	/test pull-gateway-api-inference-extension-test-unit-main

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[nirrozenbaum]

@danehans have you created a PR in istio to add it there?

[danehans]

No, just a tracker issue.