Remove inference metric Valkey produce-side code

After GraphQL and auto-scaling consumers have been migrated to UtilizationMetricService (BA-5806), remove the Valkey produce-side code that is no longer needed.

Removal targets:
- appproxy/worker/metrics.py: collect_inference_metric(), gather_inference_measures(), gather_prometheus_inference_measures() — entire metric collection pipeline
- appproxy/worker/server.py: inference_metric_collection_ctx() timer context + remove from build_root_app() cleanup_contexts list
- appproxy/worker/types.py: Circuit._app_inference_metrics, Circuit._replica_inference_metrics fields
- common/clients/valkey_client/valkey_stat/client.py: store_inference_metrics(), get_inference_app_statistics_batch(), get_inference_replica_statistics_batch()

This is a cleanup task that depends on BA-5806 being completed and verified first.

## Success Criteria
- [ ] No code path writes inference metrics to Valkey (inference.{eid}.app / inference.{eid}.replica.{rid} keys)
- [ ] No code path reads inference metrics from Valkey
- [ ] AppProxy worker starts and runs without errors after removal
- [ ] No unused imports or dead code left behind
- [ ] pants check/test passes for affected packages (appproxy, common)

JIRA Issue: BA-5807