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>

Author: database64128

unix/syscall_darwin.go

@@ -605,10 +605,6 @@ func Connectx(fd int, srcIf uint32, srcAddr, dstAddr Sockaddr, associd SaeAssocI
 const minIovec = 8
 
 func Readv(fd int, iovs [][]byte) (n int, err error) {
-	if !darwinKernelVersionMin(11, 0, 0) {
-		return 0, ENOSYS
-	}
-
 	iovecs := make([]Iovec, 0, minIovec)
 	iovecs = appendBytes(iovecs, iovs)
 	n, err = readv(fd, iovecs)
@@ -617,9 +613,6 @@ func Readv(fd int, iovs [][]byte) (n int, err error) {
 }
 
 func Preadv(fd int, iovs [][]byte, offset int64) (n int, err error) {
-	if !darwinKernelVersionMin(11, 0, 0) {
-		return 0, ENOSYS
-	}
 	iovecs := make([]Iovec, 0, minIovec)
 	iovecs = appendBytes(iovecs, iovs)
 	n, err = preadv(fd, iovecs, offset)
@@ -628,10 +621,6 @@ func Preadv(fd int, iovs [][]byte, offset int64) (n int, err error) {
 }
 
 func Writev(fd int, iovs [][]byte) (n int, err error) {
-	if !darwinKernelVersionMin(11, 0, 0) {
-		return 0, ENOSYS
-	}
-
 	iovecs := make([]Iovec, 0, minIovec)
 	iovecs = appendBytes(iovecs, iovs)
 	if raceenabled {
@@ -643,10 +632,6 @@ func Writev(fd int, iovs [][]byte) (n int, err error) {
 }
 
 func Pwritev(fd int, iovs [][]byte, offset int64) (n int, err error) {
-	if !darwinKernelVersionMin(11, 0, 0) {
-		return 0, ENOSYS
-	}
-
 	iovecs := make([]Iovec, 0, minIovec)
 	iovecs = appendBytes(iovecs, iovs)
 	if raceenabled {
@@ -706,45 +691,6 @@ func readvRacedetect(iovecs []Iovec, n int, err error) {
 	}
 }
 
-func darwinMajorMinPatch() (maj, min, patch int, err error) {
-	var un Utsname
-	err = Uname(&un)
-	if err != nil {
-		return
-	}
-
-	var mmp [3]int
-	c := 0
-Loop:
-	for _, b := range un.Release[:] {
-		switch {
-		case b >= '0' && b <= '9':
-			mmp[c] = 10*mmp[c] + int(b-'0')
-		case b == '.':
-			c++
-			if c > 2 {
-				return 0, 0, 0, ENOTSUP
-			}
-		case b == 0:
-			break Loop
-		default:
-			return 0, 0, 0, ENOTSUP
-		}
-	}
-	if c != 2 {
-		return 0, 0, 0, ENOTSUP
-	}
-	return mmp[0], mmp[1], mmp[2], nil
-}
-
-func darwinKernelVersionMin(maj, min, patch int) bool {
-	actualMaj, actualMin, actualPatch, err := darwinMajorMinPatch()
-	if err != nil {
-		return false
-	}
-	return actualMaj > maj || actualMaj == maj && (actualMin > min || actualMin == min && actualPatch >= patch)
-}
-
 //sys	connectx(fd int, endpoints *SaEndpoints, associd SaeAssocID, flags uint32, iov []Iovec, n *uintptr, connid *SaeConnID) (err error)
 //sys	sendfile(infd int, outfd int, offset int64, len *int64, hdtr unsafe.Pointer, flags int) (err error)