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

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

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

This skims down the size of os.File by 32 bytes.

Change-Id: I6b021a76f582ead5dccb29b001e7a5b068a2c2ee
Reviewed-on: https://go-review.googlesource.com/c/go/+/685416
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mark Freeman <mark@golang.org>