Remove "thread" argument in iter_unread_messages

It is unused, and no backend implemented it — so it was wrong to have it
in the function signature.
It also does not seem very helpful, we have get_threads() if we want to
do something advanced with treads.

fixes #825

modules/hds/backend.py

@@ -88,7 +88,7 @@ class HDSBackend(BaseBackend, ICapMessages):
 
         return thread
 
-    def iter_unread_messages(self, thread=None):
+    def iter_unread_messages(self):
         for thread in self.iter_threads():
             if thread.id in self.storage.get('seen', default=[]):
                 continue