Remove topic_auto_close at from PostSerializer

We should look at extracting some of the cruft from these repsonses.

app/serializers/post_serializer.rb

@@ -27,7 +27,6 @@ class PostSerializer < BasicPostSerializer
              :yours,
              :topic_id,
              :topic_slug,
-             :topic_auto_close_at,
              :display_username,
              :primary_group_name,
              :version,
@@ -72,10 +71,6 @@ class PostSerializer < BasicPostSerializer
     object.try(:topic).try(:slug)
   end
 
-  def topic_auto_close_at
-    object.try(:topic).try(:auto_close_at)
-  end
-
   def moderator?
     !!(object.try(:user).try(:moderator?))
   end