test: remove obsolete comment from dgram test

Refs: https://github.com/nodejs/node/issues/4640
PR-URL: https://github.com/nodejs/node/pull/8689


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>