util: assume modern CPU_ALLOC macros always exist

Support for the modern CPU_ALLOC macros was added 10 years ago in

  commit a73cd93b
  Author: Daniel P. Berrange <berrange@redhat.com>
  Date:   Mon Nov 16 16:08:29 2009 +0000

    Alternate CPU affinity impl to cope with NR_CPUS > 1024

This is long enough that we can assume it always exists and drop the
back compat code.

Reviewed-by: Martin Kletzander <mkletzan@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>