book2: drop template_dir variable entirely

As of the previous commit, nobody looks at it anymore.

Author: peff

lib/tasks/book2.rake

@@ -31,8 +31,6 @@ task reset_book2: :environment do
 end
 
 def genbook(code, &get_content)
-  template_dir = Rails.root.join("templates")
-
   nav = '<div id="nav"><a href="[[nav-prev]]">prev</a> | <a href="[[nav-next]]">next</a></div>'
 
   progit = get_content.call("progit.asc")