Stop logging.

In 11 years, I've never needed the log.  I have often moved files from mh folder
back to Maildir and rerun as I developed a filter.

Probably this log made sense before I added the scan-esque status line for
each message at 2005-09-02 22:36:10 +0000 .  So this has been redundant for
over 8 years.  Nice.