[Dovecot] convert plugin created maildirfolder file in root maildir directory

[Justin McAleer]

I've noticed that if a user's maildir gets created by deliver or 
pop/imap without the convert plugin running, the maildir structure is 
fine (no maildirfolder file). But, if a user gets converted, there is a 
maildirfolder file in the user's base maildir directory. As far as I can 
tell, that doesn't cause any problems, but I just wanted to mention it. 
I've looked at the code for a while but didn't find why it behaves 
differently.