dns: remove nonexistant exports.ADNAME

This error code export was mistakingly introduced in a 2012 commit which
added more error codes. The correct export.BADNAME was added in
https://github.com/nodejs/node/pull/3051.

Semver: Major
PR-URL: https://github.com/nodejs/node/pull/3051
Fixes: https://github.com/nodejs/node/issues/3050


Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>