remove unused AO CreateAsyncIteratorFromClosure

[michaelficarra]

It's unused. We should simply delete it.

[ljharb]

It was added in #2045, in the pursuit of built-in generators, for async iterator helpers. Will async iterator helpers use it? If so, it's probably best not to create more churn by removing it; if not, it should ofc be axed.

[bakkot]

Will async iterator helpers use it?

No, since the concurrency stuff they're trying to do doesn't work with generators.