Turbopack: delete broken hmr benchmark (#83882)
### What?
The benchmark is very broken:
* It doesn't await the run_once, so the code never runs
* project_path is incorrect so it doesn't find routes
* The returned Duration is never used as divan doesn't support custom benches
* `hmr_update` doesn't wait for an HMR update to happen as it just returns "no update"
* It uses `"next": "^15.0.0"` so JS and Rust code are incompatible with each other
It's easier to remove the benchmark than to fix it. It never worked anyway.
Eventually we probably want a e2e benchmark running next.js anyway.
