refactor: remove unused async collector hook (#986)

[TATP-233]

Summary

• remove the unused AsyncRunner._collector_fn abstract hook
• remove APP, off-policy, and HORA forwarding overrides plus the test stub
• retain each runner explicit _start_collector(target_fn=...) dispatch unchanged

Validation

• make test-all passed at 0fcbc2655e80299850e52d20389c132e9fd71959
• 1654 passed, 27 skipped, 273 deselected, 1 xfailed
• benchmark smoke: 33/34 module-mode and 34/35 script-mode; MLX-only entries skipped
• production tree contains no _collector_fn definition or call

Closes #986