Remove billing_events_oidc_auth feature flag

What does this MR do and why?

Removes the billing_events_oidc_auth feature flag. Billing destinations now always use BillingAuthEmitter, which authenticates requests to the DIP billing collector's protected path.

• The flag is enabled on all GitLab.com environments: dev, pre, gstg, gstg-ref (2026-07-13) and gprd (2026-09-09).
• The collector rejects unauthenticated requests, so the flag-off (unauthenticated) path cannot succeed; auth is required for every billing request.

Flag introduced in !243234 (merged).

How to set up and validate locally

Not verifiable locally (needs the GCE metadata server / Cloud Connector token and the protected collector). Auth was validated on staging and production behind the flag.

MR acceptance checklist

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