timers: cleanup extraneous property on Immediates

This was originally changed in 6f75b667
but it appears unnecessary and benhcmark results show little difference
without the extra property.

Refs: https://github.com/nodejs/node/pull/6436
PR-URL: https://github.com/nodejs/node/pull/16355


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>