internal/poll: remove rsan field from Windows' poll.operation

internal/poll: remove rsan field from Windows' poll.operation

There is no need to keep the rsan field in the poll.operation struct.

This skims down the size of os.File by 16 bytes.

Change-Id: I5e99e0e87178b63a19f0b9883b7b3d25abfd9ec3
Reviewed-on: https://go-review.googlesource.com/c/go/+/685417
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Mark Freeman <mark@golang.org>