go.sys/unix: delete nacl

It's a peculiar environment that probably doesn't belong here.
We can bring it back easily if we need it.

LGTM=dave, rsc
R=rsc, dave
CC=golang-codereviews
https://golang.org/cl/128110043

unix/mksyscall.pl

@@ -28,7 +28,6 @@ my $plan9 = 0;
 my $openbsd = 0;
 my $netbsd = 0;
 my $dragonfly = 0;
-my $nacl = 0;
 my $arm = 0; # 64-bit value should use (even, odd)-pair
 
 if($ARGV[0] eq "-b32") {
@@ -54,10 +53,6 @@ if($ARGV[0] eq "-dragonfly") {
 	$dragonfly = 1;
 	shift;
 }
-if($ARGV[0] eq "-nacl") {
-	$nacl = 1;
-	shift;
-}
 if($ARGV[0] eq "-arm") {
 	$arm = 1;
 	shift;
@@ -224,9 +219,6 @@ while(<>) {
 		$sysname = "SYS_$func";
 		$sysname =~ s/([a-z])([A-Z])/${1}_$2/g;	# turn FooBar into Foo_Bar
 		$sysname =~ y/a-z/A-Z/;
-		if($nacl) {
-			$sysname =~ y/A-Z/a-z/;
-		}
 	}
 
 	# Actual call.