dns: remove makeAsync() function check

makeAsync() is an internal method in the dns module. All of the
functions that call makeAsync() have already validated that the
callback is a function. This commit removes a redundant typeof
function check.

PR-URL: https://github.com/nodejs/node/pull/8170


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Yorkie Liu <yorkiefixer@gmail.com>