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>

lib/dns.js

@@ -56,9 +56,6 @@ function errnoException(err, syscall, hostname) {
 //   callback.immediately = true;
 // }
 function makeAsync(callback) {
-  if (typeof callback !== 'function') {
-    return callback;
-  }
   return function asyncCallback() {
     if (asyncCallback.immediately) {
       // The API already returned, we can invoke the callback immediately.