fastmodel: Use getCurrentInstCount for totalInsts().

fastmodel: Use getCurrentInstCount for totalInsts().

This had been using a custom totalInsts method on the iris
ThreadContext, but since that's equivalent to what the totalInsts
method does only through a different mechanism, we can
drop that and use getCurrentInstCount instead.

Change-Id: I058fec13e81f28285281e136635d53a2e849cb47
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22112
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>