exceptions: Remove dead code msg_format method.

zerver/lib/exceptions.py

@@ -275,6 +275,5 @@ class MissingAuthenticationError(JsonableError):
     def __init__(self) -> None:
         pass
 
-    @staticmethod
-    def msg_format() -> str:
-        return _("User is not authenticated")
+    # No msg_format is defined since this exception is caught and
+    # converted into json_unauthorized in Zulip's middleware.