remove nacl

The recent linker changes broke NaCl support
a month ago, and there are no known users of it.

The NaCl code can always be recovered from the
repository history.

R=adg, r
CC=golang-dev
https://golang.org/cl/3671042
diff --git a/test/zerodivide.go b/test/zerodivide.go
index 5fe1eb0..cd4f522 100644
--- a/test/zerodivide.go
+++ b/test/zerodivide.go
@@ -10,7 +10,6 @@
 	"fmt"
 	"math"
 	"strings"
-	"syscall"
 )
 
 type Error interface {
@@ -163,7 +162,7 @@
 func main() {
 	bad := false
 	for _, t := range errorTests {
-		if t.err != "" && syscall.OS == "nacl" {
+		if t.err != "" {
 			continue
 		}
 		err := error(t.fn)