vpnkit-forwarder: remove dead code
vpnkit-forwarder: remove dead code

The `udpEncapsulator` struct had 2 modes:

1. read from an existing `net.Conn`, unmarshal encapsulated UDP datagrams
   and feed into the proxy
2. `Accept` new connections on a `net.Listener`, unmarshal encapsulated
   UDP datagrams and feed into the proxy

The mode (2) was only useful before we switched to `Accept`ing connections
on a well-known port and multiplexing, so it's safe to delete.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@docker.com>

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit