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/aum/backend.py

@@ -240,7 +240,7 @@ class AuMBackend(BaseBackend, ICapMessages, ICapMessagesPost, ICapDating, ICapCh
 
         return thread
 
-    def iter_unread_messages(self, thread=None):
+    def iter_unread_messages(self):
         try:
             contacts = {}
             with self.browser: