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/mkall.sh

@@ -176,18 +176,6 @@ linux_arm)
 	mksysnum="curl -s 'http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/plain/arch/arm/include/uapi/asm/unistd.h' | ./mksysnum_linux.pl"
 	mktypes="GOARCH=$GOARCH go tool cgo -godefs"
 	;;
-nacl_386)
-	mkerrors=""
-	mksyscall="./mksyscall.pl -l32 -nacl"
-	mksysnum=""
-	mktypes=""
-	;;
-nacl_amd64p32)
-	mkerrors=""
-	mksyscall="./mksyscall.pl -nacl"
-	mksysnum=""
-	mktypes=""
-	;;
 netbsd_386)
 	mkerrors="$mkerrors -m32"
 	mksyscall="./mksyscall.pl -l32 -netbsd"