events: pass the original listener added by once

When removing a `once` listener, the listener being passed to
the `removeListener` callback is the wrapper. This unwraps the
listener so that `removeListener` is passed the actual listener.

PR-URL: https://github.com/nodejs/node/pull/6394


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>