Remove dead predict_fn code path from service
Production always uses orchestrator. The predict_fn/async_predict_fn
path was only used in tests and never ran in production.
- Remove PredictFn, AsyncPredictFn, PredictFuture types
- Remove predict_via_function method
- Remove legacy_pool field
- Remove PredictionService::new(pool) constructor
- Enforce READY requires orchestrator (silently ignores otherwise)
- Update tests to not create impossible states
