cpu: Get rid of the nextInstEventCount method.

cpu: Get rid of the nextInstEventCount method.

This was only used by the KVM CPU, and it has access to all it needs to
figure out that value locally without requiring all the ThreadContexts
to implement an equivalent function.

Change-Id: I17a14ce669db2519edf129db761ebd8dc3bd4129
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22114
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>