Remove an unused import

Signed-off-by: Wieland Hoffmann <themineo@gmail.com>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/mbnames.py

@@ -21,10 +21,6 @@ import json
 from threading import Lock
 from os import listdir, makedirs, path, unlink
 from sys import exc_info
-try:
-    import UserDict
-except ImportError: # Py3.
-    from collections import UserDict
 try:
     from ConfigParser import NoSectionError
 except ImportError: # Py3.