syscall: remove sysctl workaround for openbsd

Now that Go will no longer work on OpenBSD versions prior to 5.1,
remove the sysctl workaround that was needed for 5.0 and earlier.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/6116050

src/pkg/syscall/syscall_bsd.go

@@ -13,7 +13,6 @@
 package syscall
 
 import (
-	"runtime"
 	"unsafe"
 )
 
@@ -553,16 +552,7 @@ func Sysctl(name string) (value string, err error) {
 		return "", err
 	}
 	if n == 0 {
-		// TODO(jsing): Remove after OpenBSD 5.2 release.
-		// Work around a bug that was fixed after OpenBSD 5.0.
-		// The length for kern.hostname and kern.domainname is always
-		// returned as 0 when a nil value is passed for oldp.
-		if runtime.GOOS == "openbsd" && (name == "kern.hostname" || name == "kern.domainname") {
-			// MAXHOSTNAMELEN
-			n = 256
-		} else {
-			return "", nil
-		}
+		return "", nil
 	}
 
 	// Read into buffer of that size.