cpu, sim: Remove unused System::totalNumInst

cpu, sim: Remove unused System::totalNumInst

This counter gets augmented for every executed instruction but it
is not used. It is also overlapping with the

BaseCPU::numSimulatedInsts

A client willing to know the number of simulated instruction should rely
on the interface above.

Change-Id: Ic5c805ac3b2e87bbacb365108d4060f53e044b4e
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/25305
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>