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/phpbb/backend.py

@@ -72,7 +72,6 @@ class PhpBBBackend(BaseBackend, ICapMessages, ICapMessagesPost):
                 thread = Thread(url2id(link.url))
                 thread.title = ('%s ' % root_link.title if root_link else '') + link.title
                 thread.date = link.date
-                thread.nb_messages = link.nb_messages
                 thread.flags = thread.IS_DISCUSSION
                 yield thread