debugger: remove unneeded callback check

In `lib/_debugger.js`, remove check for `cb` in line 571 as it is
guaranteed to be truthy due to line 521.

PR-URL: #5319
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/_debugger.js

@@ -568,7 +568,7 @@ Client.prototype.mirrorObject = function(handle, depth, cb) {
 
  waitForOthers();
  function waitForOthers() {
- if (--waiting === 0 && cb) {
+ if (--waiting === 0) {
  keyValues.forEach(function(kv) {
  mirror[kv.name] = kv.value;
  });