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/phpbb/backend.py

@@ -130,7 +130,7 @@ class PhpBBBackend(BaseBackend, ICapMessages, ICapMessagesPost):
                        children=[],
                        flags=Message.IS_HTML)
 
-    def iter_unread_messages(self, thread=None):
+    def iter_unread_messages(self):
         with self.browser:
             url = self.browser.get_root_feed_url()
             for article in Newsfeed(url, rssid).iter_entries():