sim: Delete an unused member in the System class.

sim: Delete an unused member in the System class.

This was supposed to be deleted as part of the change titled:
arch,cpu,dev,sim,mem: Collect System thread elements into a subclass.

but it was left out of the checked in version somehow.

Change-Id: I0dbb0b4fa6ae29649a80d1cb883e48ad50116c31
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/30194
Maintainer: Gabe Black <gabeblack@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>