timers: do not expose .unref()._handle._list

The list is defunct at this point.
I believe this to be in effect a minor defect from
3eecdf9f

PR-URL: https://github.com/nodejs/node/pull/8422


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Julien Gilli <jgilli@nodejs.org>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>