Remove dead header and a redundant edge record

[nclack]

Two small, behavior-neutral cleanups flagged during the orchestration-migration reviews (#143/#148/#149/#151).

• Remove dead flush.helpers.h. The header had no remaining users after the scheduler work; deleted the file, its two #includes, and the CMakeLists entry. Verified no symbol it declared is referenced anywhere.
• Drop a redundant POOL_FILLED record. On the drain-after-kick (multiarray sync) schedule, schedule_accumulate_epoch recorded the pool-filled edge and then schedule_flush_accumulated recorded it again on the same stream. Skip the first on that path; the flush-accumulated release is the load-bearing one before its kick. The pipelined and drain-before-kick paths are unchanged.

Validation (L40, sm_89, RelWithDebInfo): ctest -E "(s3)" 48/48; zero new warnings.

[codecov]

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