chore(entity-resolver): remove dead resolve_entity/_create_entity/link_unit_to_entity

[nicoloboschi]

Summary

Removes four dead methods from EntityResolver that have no live callers:

• resolve_entity (singular) — superseded by the batched resolve_entities_batch path used by retain/PATCH; zero callers anywhere (incl. tests).
• _create_entity — only reachable from the dead resolve_entity.
• link_unit_to_entity (singular) — superseded by link_units_to_entities_batch; zero callers.
• _update_cooccurrence — only reachable from the dead link_unit_to_entity.

Why

While reviewing #2353 (sort unit_ids to avoid a concurrent-insert deadlock on graph_maintenance_queue), _create_entity's entities ... ON CONFLICT ... DO UPDATE looked like another live concurrent-deadlock site. It isn't — it's unreachable. The live entity paths are already deadlock-safe by consistent lock ordering:

• entity create (bulk_insert_entities) — caller sorts by lower(name) + DO NOTHING
• entity-stats UPDATE & co-occurrence DO UPDATE (flush_pending_stats) — both sorted() with explicit "prevents deadlocks" comments
• bulk_insert_links, bulk_insert_unit_entities — sorted at the caller

Deleting the dead code so the misleading unsorted DO UPDATE stops tripping up future readers and reviewers.

Scope / safety

• Pure deletion (230 lines), no behavior change.
• ruff check/format and ty clean; lint.sh passes.
• Entity-resolver suite green locally: test_entity_resolver.py, test_entity_labels.py, test_entity_resolver_pg_trgm.py, test_graph_entity_fanout_cap.py — 106 passed.
• These methods were already flagged advisory-only by vulture (check-unused-code job).