Closing the openpgp working group

Hi, folks.  Back in January Derek started a thread on a new charter.
That never really came to any conclusion.  I had given Derek a
deadline to produce a charter that is well past.  So, I'm going to
close the openpgp working group.

If people ever do get a concrete proposal together, please don't
hesitate to contact the security ADs at the time this happens and
propose a new working group.