Revert "[hipGraph] Add graph signal p

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## Summary

Reverts #5333 (`f17b05703a`) due to two issues observed in `GraphBench`:

1. **Resource leak.** Per-launch `GraphSignalPool` was deleted via a
forward-declared pointer in `~AccumulateCommand`, so `~GraphSignalPool`
never ran and every per-launch HSA signal was leaked (`Resource leak
detected by SharedSignalPool, N Signals leaked`).
2. **Per-launch host overhead.** Shifting from the runtime's rotating
`signal_list_` (which reuses signals after warmup with `WaitCurrent` +
`WaitNext`) to a per-launch graph signal pool reintroduces N×
`signal_create` + `signal_destroy` on every `hipGraphLaunch`, causing
measurable regression in `graph_bench` across `straight`, `paths2`,
`paths4`, `full2`, `full4` topologies.

A safer redesign that preserves PR #5333's per-launch isolation while
recycling signals (mirroring the runtime pool's `WaitCurrent`/`WaitNext`
discipline) is needed before re-landing.

## Test plan

- [ ] `graph_bench` runs to completion across all five topologies with
no `Signals leaked` warning.
- [ ] `graph_bench` per-launch numbers match pre-PR-#5333 baseline.
- [ ] Existing `hipGraph` unit tests still pass.

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Co-authored-by: Cursor <cursoragent@cursor.com>
[rocm-systems] ROCm/rocm-systems#5951 (commit a23a24f)