unix: drop pre-1.10 compatibility in TestSCMCredentials

The last pre-1.10 release - Go 1.9 - was release 2017-08-24 and is no
longer supported since Go 1.11 was released 2018-08-24.

This effectively reverts https://golang.org/cl/80275

Updates golang/go#22899

Change-Id: Id7ab3f6b6d6430977db9345def383d875396e1e7
Reviewed-on: https://go-review.googlesource.com/c/sys/+/292969
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/creds_test.go

@@ -9,7 +9,6 @@ package unix_test
 
 import (
 	"bytes"
-	"go/build"
 	"net"
 	"os"
 	"testing"
@@ -36,11 +35,6 @@ func TestSCMCredentials(t *testing.T) {
 	}
 
 	for _, tt := range socketTypeTests {
-		if tt.socketType == unix.SOCK_DGRAM && !atLeast1p10() {
-			t.Log("skipping DGRAM test on pre-1.10")
-			continue
-		}
-
 		fds, err := unix.Socketpair(unix.AF_LOCAL, tt.socketType, 0)
 		if err != nil {
 			t.Fatalf("Socketpair: %v", err)
@@ -123,13 +117,3 @@ func TestSCMCredentials(t *testing.T) {
 		}
 	}
 }
-
-// atLeast1p10 reports whether we are running on Go 1.10 or later.
-func atLeast1p10() bool {
-	for _, ver := range build.Default.ReleaseTags {
-		if ver == "go1.10" {
-			return true
-		}
-	}
-	return false
-}