unix: drop fallback to accept in Accept on Linux

Same as CL 346849 does for package syscall.

The minimum required Linux kernel version for Go 1.18 will be changed to
2.6.32, see golang/go#45964. The current minimum required version is
2.6.23 and accept4 was added in 2.6.28, so the fallback to accept in
Accept on Linux can be removed.

For golang/go#45964

Change-Id: Idf915d5a24cf570a2cfa0f0548c1841f3fc64d5a
Reviewed-on: https://go-review.googlesource.com/c/sys/+/347329
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matt Layher <mdlayher@gmail.com>
diff --git a/unix/syscall_linux.go b/unix/syscall_linux.go
index c16108a..60d6c16 100644
--- a/unix/syscall_linux.go
+++ b/unix/syscall_linux.go
@@ -1209,11 +1209,7 @@
 func Accept(fd int) (nfd int, sa Sockaddr, err error) {
 	var rsa RawSockaddrAny
 	var len _Socklen = SizeofSockaddrAny
-	// Try accept4 first for Android, then try accept for kernel older than 2.6.28
 	nfd, err = accept4(fd, &rsa, &len, 0)
-	if err == ENOSYS {
-		nfd, err = accept(fd, &rsa, &len)
-	}
 	if err != nil {