Fix total_comments indexer

"@property removes Acquisition wrappers from 'self'.
I don't remember why exactly (some implementation detail of Acquisition),
but it's a fact of life."

Quote from @davisagli on
https://github.com/plone/plone.app.discussion/pull/58

In short: removing the @property from total_comments fix the problem.

plone/app/discussion/conversation.py

@@ -77,7 +77,6 @@ class Conversation(Traversable, Persistent, Explicit):
         parent = aq_inner(self.__parent__)
         return parent.restrictedTraverse('@@conversation_view').enabled()
 
-    @property
     def total_comments(self):
         public_comments = [
             x for x in self.values()