unix: remove redundant xnu version check for {p}readv/{p}writev

As discussed later in golang/go#64710, these checks are unnecessary.

For golang/go#64710

Change-Id: I8693af81cc543b886732922eb5cd64e07ed4b7a8
Reviewed-on: https://go-review.googlesource.com/c/sys/+/688858
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Mark Freeman <mark@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>