os: remove unused import to fix build

Change-Id: Ia8c1c77590115a5ffda144962436d489ed77a423
Reviewed-on: https://go-review.googlesource.com/18227Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/pipe_test.go

@@ -13,7 +13,6 @@ import (
 	"os"
 	osexec "os/exec"
 	"os/signal"
-	"runtime"
 	"syscall"
 	"testing"
 )