alpha,arm,mips,power,riscv,sparc,x86: Get rid of TheISA::NoopMachInst.

alpha,arm,mips,power,riscv,sparc,x86: Get rid of TheISA::NoopMachInst.

It's no longer used.

Change-Id: I4a71bcb214f1bb186b92ef50841eca635e6701c5
Reviewed-on: https://gem5-review.googlesource.com/6826
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>