dgram: remove unnecessary fd property from Socket

It seems the property socket.fd is used long before for the
compatibility reason but it's not used anymore. It's also
not documented in docs so that it's not a public api/property.
Keep the property also make the code a bit confusing as
socket._handle.fd is the actual property representing fd.

PR-URL: https://github.com/nodejs/node/pull/21684


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>