Remove `Note#noteable_type_name`

This method was added in Dec 2012, its single use was removed two days
later, and it's been there ever since.

app/models/note.rb

@@ -323,10 +323,6 @@ class Note < ActiveRecord::Base
     noteable
   end
 
-  def noteable_type_name
-    noteable_type.downcase if noteable_type.present?
-  end
-
   # FIXME: Hack for polymorphic associations with STI
   #        For more information visit http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#label-Polymorphic+Associations
   def noteable_type=(noteable_type)