Don't store a link to message we're replying to

* gnorb-gnus.el (gnorb-gnus-outgoing-do-todo): When calling this
  function on an outgoing/sent message, don't go back and store a link
  to the original message we replied to.

This was a holdover from the earlier system of links and properties. It
slows everything down, and blinks the window annoyingly. What should
happen instead is that a registry association is made for the message
we're replying to. That can be done later.