Cleaning up bogus completion buffers [was newsgroups]

[Harry Putnam <reader@newsguy.com>]

ShengHuo ZHU <zsh@cs.rochester.edu> writes:

> You could browse the server to find out new groups.  
> 
> Now, I've changed the behavior of `C-k', that is, if a group is killed
> twice, it will no longer be in gnus-killed-list.

A related question:  I've had occasion to misstype an entry when
using `j' to jump to a group.

Imagine ... you type  `j' nnml:newestgroup  and you really meant
nnml:newergroup.

The result will be that a killed group with the mistaken name is
created at the bottom of group buffer.  Ok.. fine, you can kill it
again.

Now here's the rub.  That mistaken, twice killed group is now included
in completion buffers.

If you type nnml:n<SPC> you will see it there.  One would think that
the info that appears in a completion buffer would be stored somewhere
and therefore editable, but grepping .newsrc.eld for the phantom
`newestgroup' will prove futile.