Remove legacy consent_store KV persistence

[ChristianPavilonis]

Summary

Remove the legacy consent_store KV persistence path and make ec_identity_store the only KV-backed EC/consent lifecycle store.

Current deployments can run without consent_store, and normal request/auction flows do not read from it. Consent decisions are built from request cookies/headers/geo in EcContext, while identity state and withdrawal tombstones live in ec_identity_store.

Current behavior

• consent_store is still exposed in config:
  • crates/trusted-server-core/src/consent_config.rs
  • trusted-server.toml
  • fastly.toml
• The consent pipeline still has legacy KV fallback/write helpers:
  • crates/trusted-server-core/src/consent/mod.rs
  • crates/trusted-server-core/src/storage/kv_store.rs
• Normal EC context creation passes kv_store: None, so the consent KV fallback/write path is not active:
  • crates/trusted-server-core/src/ec/mod.rs
• Auction uses in-memory request consent from EcContext, not consent_store:
  • crates/trusted-server-core/src/auction/endpoints.rs
• The only live consent_store touchpoint appears to be best-effort deletion during consent withdrawal:
  • crates/trusted-server-core/src/ec/finalize.rs

Proposed cleanup

1. Remove consent_store from consent configuration and sample/local config.
2. Remove consent KV fallback/write plumbing from ConsentPipelineInput and build_consent_context.
3. Remove or quarantine crates/trusted-server-core/src/storage/kv_store.rs if it has no remaining callers.
4. Remove best-effort consent_store deletion from EC finalization; keep ec_identity_store withdrawal tombstones as the authoritative revocation marker.
5. Update/remove tests that assume a configured/missing consent_store behavior.
6. Update docs/comments to clarify:
   • live consent comes from request cookies/headers/geo;
   • EC identity lifecycle and withdrawal state live in ec_identity_store;
   • there is no separate consent KV store.

Non-goals

• Do not add all legacy consent-store fields to ec_identity_store unless there is a concrete product requirement for persisted consent fallback.
• Do not change auction consent gating behavior.
• Do not change EC tombstone semantics.

Notes

ec_identity_store currently stores a minimal consent snapshot (tcf, gpp, ok, updated) plus identity graph state. The legacy consent_store stored additional fields such as us_privacy, gpc, jurisdiction, gpp_section_ids, raw_ac_string, and a fingerprint. Those are still important as live request consent concepts, but they are not currently used as persisted runtime state.