internal/poll: assume we have CancelIoEX on Windows
As of the Go 1.11 release we require at least Windows 7, so CancelIoEx
is always available. This lets us simplify the code to not require
dedicated threads to handle I/O requests.
Fixes #37956
Change-Id: If1dc4ac4acb61c43e4f2a9f26f225869050262a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/225060
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Alex Brainman <alex.brainman@gmail.com>