threadutil: remove broken os.exit()

We are missing the import of 'os' python module since commit d839be3c61
(Sat Apr 16 20:33:35 2005 +0100) which was when John switched from SVN to Git.

Happily, it help us today: we still had no feedback for this missing import,
6 years later.  So, we can remove the os.exit() call safely.

That beeing said, we still don't know if the above sys.exit() was ever touched.
My guess is that it never was. Keep this (hopefully) commented statement to
ensure the thread terminate and not play too much with the Murphy's law. :-)

Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/threadutil.py

@@ -122,12 +122,10 @@ def threadexited(thread):
             raise SystemExit
         ui.threadException(thread)      # Expected to terminate
         sys.exit(100)                   # Just in case...
-        os._exit(100)
     elif thread.getExitMessage() == 'SYNC_WITH_TIMER_TERMINATE':
         ui.terminate()
         # Just in case...
         sys.exit(100)
-        os._exit(100)
     else:
         ui.threadExited(thread)