Remove Deferred.asWorker() converter function

[zach-klippenstein]

This converter looks a lot like Single.asWorker(), but has a major pitfall: it doesn't scope. This difference is subtle, and might make code hard to reason about: given foo.asWorker(), the work will be scoped to the Worker if foo is a cold future like Single, but not if it's a hot future like Deferred.

A more theoretical reason is that I think it's bad coroutines practice to expose Deferreds as public API because it breaks structured concurrency. We shouldn't encourage that practice by providing this converter.

[rjrjr]

This was done.