cpufreq: stats: drop 'cpu' field of struct cpufreq_stats

'cpu' field of struct cpufreq_stats isn't used anymore and so can be dropped.
This change makes cpufreq_stats_update_policy_cpu() empty and so that is removed
as well.

Reviewed-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>