timers: fix subsequent enroll calls not working

A bug was introduced in #17704 which meant that subsequent
calls to enroll would unset the new _idleTimeout and the
enrolled object could never again function as a timer.

PR-URL: https://github.com/nodejs/node/pull/19936


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>