test(turbopack): remove dead top-level task unmarks (#98246)
## Summary
Remove stale
`unmark_top_level_task_may_leak_eventually_consistent_state()` calls
from tests and a benchmark that already use strongly consistent
operation-root reads.
## Verification
- `cargo test -p turbo-tasks-backend --test detached --test eviction`
- `cargo check -p turbo-tasks-backend --benches`
- `cargo fmt --all -- --check`
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Co-authored-by: vercel-fleet-prod[bot] <318278635+vercel-fleet-prod[bot]@users.noreply.github.com>
Co-authored-by: Luke Sandberg <210140+lukesandberg@users.noreply.github.com>
