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>

lib/dns.js

@@ -330,7 +330,6 @@ exports.NOTFOUND = 'ENOTFOUND';
 exports.NOTIMP = 'ENOTIMP';
 exports.REFUSED = 'EREFUSED';
 exports.BADQUERY = 'EBADQUERY';
-exports.ADNAME = 'EADNAME';
 exports.BADNAME = 'EBADNAME';
 exports.BADFAMILY = 'EBADFAMILY';
 exports.BADRESP = 'EBADRESP';