arch,cpu,sim: Eliminate the now empty kernel statistics classes.

arch,cpu,sim: Eliminate the now empty kernel statistics classes.

This includes the base and ISA specific Kernel::Statistics classes, the
plumbing through ThreadContext to access them, and the switching
header file associated with them.

Change-Id: Ia511a59325b629aa9ccc0e695ddd47ff11916499
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/25149
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>