fastmodel: Get rid of the back channel mem port in FastModel::ArmCPU.

fastmodel: Get rid of the back channel mem port in FastModel::ArmCPU.

This was to support port proxies and getInstPort and getDataPort. With
some recent upstream changes, getInstPort and getDataPort are only used
for CPU switching which we can't support (TLM ports are bound
permanently), and with the sendFunctional delegate for port proxies,
we don't need to have a traditional gem5 port lying around.

This gets rid of the "mem" port and all its plumbing.

Change-Id: Ic68a40a26b24aa05b33da0510c9f4b7621cbf578
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21048
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>