unload handler logs warnings in chrome

https://github.com/cloudflare/partykit/blob/f0a2e97d233f24545b2648aec2ed6a191e11074e/packages/y-partyserver/src/provider/index.ts#L398

```
[Violation] Permissions policy violation: unload is not allowed in this document.
```

Start at packages/y-partyserver/src/provider/index.ts around line 398, where the unload handler is referenced. Reproduce the issue in Chrome and inspect how that handler is registered or used. Done means the handler no longer produces the Permissions policy violation warning in Chrome.

Written by the indexing model from the issue text.