unix: remove nacl build tag from dirent code

Although CL 128110043 had deleted nacl from x/sys/unix, CL 38758
accidentially introduced its build tag in dirent.go when copying
over stdlib syscall code, which this CL removes.

Fixes golang/go#33391.

Change-Id: Iad6fe9818e38f14548eecdc078392865a60e4685
Reviewed-on: https://go-review.googlesource.com/c/sys/+/188361
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/dirent.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build aix darwin dragonfly freebsd linux nacl netbsd openbsd solaris
+// +build aix darwin dragonfly freebsd linux netbsd openbsd solaris
 
 package unix