template: delete old template code.

It's already in old/template; make that build.
Update a couple of references to point to the old template.
They can be updated later.
Update goplay to use exp/template.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/4902046

doc/codelab/wiki/htmlify.go

@@ -1,8 +1,8 @@
 package main
 
 import (
+	"old/template"
 	"os"
-	"template"
 	"io/ioutil"
 )