doc/asyncworker.md: remove AsyncProgressQueueWorker::ExecutionProgress::Signal
doc/asyncworker.md: remove AsyncProgressQueueWorker::ExecutionProgress::Signal

`AsyncProgressQueueWorker::ExecutionProgress::Signal()` was never supported.
It was removed in f3cdf8d2ed89728b6994dbb568b902ea80203177

Update the markdown docs to reflect this.

nodejs/nan,nodejs/nan,nodejs/nan,nodejs/nan