sim,cpu: Get rid of the unused instEventQueue.

sim,cpu: Get rid of the unused instEventQueue.

This queue was set up to allow triggering events based on the total
number of instructions executed at the system level, and was added in
a change which added a number of things to support McPAT. No code
checked into gem5 actually schedules an event on that queue, and no
code in McPAT (which seems to have gone dormant) either downloadable
from github or found in ext modify gem5 in a way that makes it use
the instEventQueue.

Also, the KVM CPU does not interact with the instEventQueue correctly.
While it does check the per-thread instruction event queue when
deciding how long to run, it does not check the instEventQueue. It will
poke it to run events when it stops for other reasons, but it may (and
likely will) have run beyond the point where it was supposed to stop.

Since this queue doesn't seem to actually be used for anything, isn't
being used properly in all cases anyway, and adds overhead to all the
CPU models, this change eliminates it.

Change-Id: I0e126df14788c37a6d58ca9e1bb2686b70e60d88
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21783
Maintainer: Gabe Black <gabeblack@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Tiago Mück <tiago.muck@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>