cpu: Access inst events through ThreadContext instead of the CPU.

cpu: Access inst events through ThreadContext instead of the CPU.

Also delete the CPU interface.

Change-Id: I62a6b0a9a303d672f4083bdedf393f9f6d07331f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22109
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>