Removed the built-in FTP client.

That's another of these things that are not the MUA's tasks.
There are good FTP clients around, just use one of them.
We could provide an external FTP wrapper script around wget or so,
to use with nmh-access-ftp. However, I'm not sure how to deal with
message/external-body in general. Although this concept is great,
I've never seen any such message in all my email life.