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

weboob/capabilities/messages.py

@@ -105,8 +105,6 @@ class Thread(CapBaseObject):
         self.add_field('root', Message)
         self.add_field('title', basestring)
         self.add_field('date', (datetime.datetime, datetime.date))
-        self.add_field('nb_messages', int)
-        self.add_field('nb_unread', int)
         self.add_field('flags', int, self.IS_THREADS)
 
     def iter_all_messages(self):