chore(ol-istio): removed unused gateway schema values

General MR

Summary

This MR removes the unused istioGatewayPublic and istioGatewayPassthrough field definitions from the Big Bang values schema. These were part of the earlier operatorless istio work and are no longer needed.

I ran two pipelines, one with istioCore enabled and one without. Both passed.

Linked Issue

Closes #2573 (closed)

Upgrade Notices

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