WebApp.addHtmlAttributeHook hooks are not cleared on HMR updates

When calling a `WebApp.addHtmlAttributeHook`, previous hooks are not cleared upon HMR updates.

It's worth noting that this doesn't seem to any negative impact on my app.

Test done locally:

```
// ...

const randomNumber = Math.random()

WebApp.addHtmlAttributeHook(req => {
console.log(randomNumber)

return null
})
```

Here is what the console returns after 2 updates on each request:

```
0.6168036561182655
0.06950770055811595
0.5245338011658636
```

Reproduce the issue with WebApp.addHtmlAttributeHook and two HMR updates, checking the logged random values on a request. Then trace the hook registration and HMR update entry points; done means prior hooks no longer run after an update, while the current hook still runs.

Written by the indexing model from the issue text.