Cleanup `alwaysThrottleDisappearingFallbacks` Flag (#28639)

## Summary

After realizing that this feature flag is entangled with
`alwaysThrottleRetries`, we're going to undo
https://github.com/facebook/react/pull/28550

## How did you test this change?

```
$ yarn test
$ yarn flow dom-browser
$ yarn flow dom-fb
$ yarn flow fabric
```