internal/poll: remove qty and flags fields from Windows' poll.operation

internal/poll: remove qty and flags fields from Windows' poll.operation

There is no need to keep the qty and flags fields in the poll.operation
struct.

This skims down the size of os.File by 16 bytes and makes poll.operation
harder to misuse.

Change-Id: I8943d88f29ed3c7eefbb83114b0d31052abbe646
Reviewed-on: https://go-review.googlesource.com/c/go/+/685436
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Mark Freeman <mark@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>