sim: Remove check whether the System port is connected.

sim: Remove check whether the System port is connected.

The port will report an error if something tries to use it and
it's not connected. If it isn't needed, there's no reason to force
users to hook something up to it just to satisfy the check.

Change-Id: I0668b8a86c8cb323aba51670fb7914d35acc5198
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34815
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>