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: https://github.com/nodejs/node/pull/5319


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