cpufreq: Remove the ->stop_cpu() driver callback

Now that all users of ->stop_cpu() have been migrated to using other
callbacks, drop it from the core.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
[ rjw: Minor edits in the subject and changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>