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

test/env.go

@@ -1,4 +1,3 @@
-// [ $GOOS != nacl ] || exit 0  # NaCl runner does not expose environment
 // $G $F.go && $L $F.$A && ./$A.out
 
 // Copyright 2009 The Go Authors. All rights reserved.