Remove nb_unread/nb_messages in CapMessages

Most backends don't set it nb_messages and no application used it.
No backends set nb_unread and only one application tried to use it.

This should help avoid useless fillobj() calls in some cases.

fixes #827

modules/hds/backend.py

@@ -47,7 +47,6 @@ class HDSBackend(BaseBackend, ICapMessages):
                 thread = Thread(story.id)
                 thread.title = story.title
                 thread.date = story.date
-                thread.nb_messages = 1
                 yield thread
 
     GENDERS = ['<unknown>', 'boy', 'girl', 'transexual']