Revert "cluster: wait on servers closing before disconnect"
This reverts commit 9c0a1b8c.
CI is timing out, work is continuing in https://github.com/nodejs/io.js/pull/1934
I merged https://github.com/nodejs/io.js/pull/1400 without running CI on it, and after removing what seemed an irrelevant setTimeout(). Sorry, I won't do that again.