Remove or implement the dead `discoveryIndexUrl` field

[JSONbored]

Problem: The extension has a discoveryIndexUrl config field that is self-acknowledged as dead in the extension's own README — it does nothing today.

Area: AMS / Extension

Proposal: Either remove the field entirely, or implement what it was originally meant to do (pointing at the not-yet-implemented hosted discovery plane) — whichever the maintainer prefers; default to removing it if the hosted discovery plane's fate (see the ORB Cloud Readiness milestone context) is still undecided.

Deliverables:

• No dead, documented-as-dead config fields remaining in the extension.

[linear-code]

JSO-3521