cpufreq: Remove (now) unused 'last_cpu' from struct cpufreq_policy

'last_cpu' was used only from cpufreq-stats and isn't used anymore. Get rid of
it.

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>