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>

test/parallel/test-dgram-oob-buffer.js

@@ -17,4 +17,4 @@ socket.send(buf, 3, 1, common.PORT, '127.0.0.1', ok);
 // Since length of zero means nothing, don't error despite OOB.
 socket.send(buf, 4, 0, common.PORT, '127.0.0.1', ok);
 
-socket.close(); // FIXME should not be necessary
+socket.close();