cpu: Get rid of load count based events.

cpu: Get rid of load count based events.

This was initially added in 2003 and only supported in the simple CPUs.
It's oddly specific since there are no other similar event queues for,
for instance, stores, branches, system calls, etc.

Given that this seems like a historical oddity which is only partially
supported and would be very hard to support on more diverse CPU types
like KVM or fast model which don't generally have hooks for counts of
specific instruction types.

Change-Id: I29209b7ffcf896cf424b71545c9c7546f439e2b9
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21780
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>