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

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

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

This skims down the size of os.File by 112 bytes.

Change-Id: I5c7b1f3989f9bb5f1748df2cba8128d9c479b35d
Reviewed-on: https://go-review.googlesource.com/c/go/+/685418
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>