internal/poll: remove handle field from Windows' poll.operation
The handle field can be accessed directly wherever needed, there is
no need to store it in the operation struct.
This skims down the size of os.File by 16 bytes.
Change-Id: I87c94cb773437891127b6c36dc7f8883622ffed3
Reviewed-on: https://go-review.googlesource.com/c/go/+/685435
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com> Reviewed-by: Damien Neil <dneil@google.com>