Remove installation of the dragonfly{,.submit}.cf files since users may

Remove installation of the dragonfly{,.submit}.cf files since users may
do a 'make upgrade' before a 'make buildworld' and the .cf files won't
exist.  Strictly speaking, we don't need them in /etc/mail/ for
/etc/mail/Makefile to work properly anyway so this is a safe change.

I prefer this to adding the .cf files as dependencies to the upgrade
target since that target shouldn't do any building (i.e., 'make upgrade'
doesn't do things like 'make obj' first).