timers: remove unused variable

A recent commit removed the usage of the second argument of
tryOnTimeout but left the definition in place. Remove it.

PR-URL: https://github.com/nodejs/node/pull/18579


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>