udp: remove a needless instanceof Buffer check

When a string is passed to udpsock.send, it is automatically
converted to a Buffer. In that case, it is no longer needed
to test whether or not the argument is a Buffer or not.

PR-URL: https://github.com/nodejs/node/pull/4301


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Roman Reiss <me@silverwind.io>