Remove one-time cleanup of legacy `activeDiscoveryProviderIds` globalState key (~6 months after 0.9.2)

Background

The pre-0.9.0 Service Discovery visibility model used an opt-in allow-list stored in the activeDiscoveryProviderIds globalState key. Since v0.9.0 the model is an opt-out deny-list (hiddenDiscoveryProviderIds) and the legacy key is no longer read (see src/services/discoveryProviderVisibility.ts).

The upgrade is intentionally non-migrated (opt-in → opt-out; documented inline), but the legacy key was left behind as dead data. To clean it up, PR #832 (milestone 0.9.2) added a one-time, best-effort cleanup that deletes the stale key on activation.

What to do (this issue)

Once the vast majority of users have upgraded past 0.8.x — target a release ~6 months out (around early 2027) — remove the temporary cleanup so it doesn't run forever:

• Delete removeLegacyActiveDiscoveryProviderIds() in src/services/discoveryProviderVisibility.ts.
• Remove its call in ClustersExtension.registerDiscoveryServices() (src/documentdb/ClustersExtension.ts).
• Remove the LEGACY_ACTIVE_DISCOVERY_PROVIDER_IDS_KEY constant and any dedicated test.

Context

• Cross-version upgrade behavior and the deliberate no-migration decision are documented in discoveryProviderVisibility.ts.
• Introduced by #832.

Read the upgrade notes and cleanup implementation in src/services/discoveryProviderVisibility.ts, then inspect ClustersExtension.registerDiscoveryServices() in src/documentdb/ClustersExtension.ts. Remove the temporary cleanup call, function, constant, and dedicated test described in the issue. Run the relevant test suite and verify no references to the legacy key remain.