nntpd: remove redundant {groups} shortcut

nntpd: remove redundant {groups} shortcut

It's not worth confusing hackers reading the source to have
two ways to access the same (large) hash table.  So just
go through PublicInbox::Config objects for now since the
extra hash lookup isn't going to be noticeable.

I've also started favoring "for" instead of "foreach"
since they're the equivalent perlop and less wear on
my fingers + keyboard.