dns: improve callback performance

It appears that either c-ares no longer calls callbacks synchronously
or we have since explicitly taken care of the scenarios in which
c-ares would call callbacks synchronously (e.g. resolving an IP
address or an empty hostname). Therefore we no longer need to have
machinery in place to handle possible synchronous callback invocation.
This improves performance significantly.

PR-URL: https://github.com/nodejs/node/pull/13261


Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: James M Snell <jasnell@gmail.com>