remove net/pop3gwd

I propose removal of net/pop3gwd for the following reasons:

- Website is down
- Not sure it's useful nowadays (who needs a POP3 proxy?)
- It's a network daemon facing the Internet, not updated since 23 years
(last release was in 1997 per the HISTORY file in sources)

pkg/DESCR

This program is an application-level gateway, or proxy, designed to
allow mail retrieval from POP3 servers by systems that either:

(a) are behind a firewall or screening router OR
(b) do not have an assigned IP number OR
(c) must share a SLIP/PPP connection on another system

It connects to the POP3 server on behalf of the client, performs the
login and then passes data both ways until either the client or the
server shut down the connection. In the meanwhile, the connection will
appear to have originated on the host running the proxy.