cpu: Stop checking for PC changes when servicing a PCEventQueue.

cpu: Stop checking for PC changes when servicing a PCEventQueue.

First of all, this would arbitrarily skip events based on when they
were encountered in the queue. Second, this is one of the three places
where the ThreadContext is actually accessed in pc_event.cc. By
removing this and the other uses, this file can be included even when
using the NULL ISA, and a lot of #ifdefs can be removed.

Change-Id: If81f5e9ff9d3f9833145fec0b6062b4bda8d2e47
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22100
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>