cleanup: remove redundant import

For some reason the import checker only caught this on py3.

Differential Revision: https://phab.mercurial-scm.org/D6657

tests/test-bookmarks-corner-case.t

@@ -119,7 +119,6 b' We build a server side extension for thi'
   > import atexit
   > import os
   > import time
-  > import atexit
   > from mercurial import error, extensions, bookmarks
   > 
   > def wait(repo):