cpu_cooling: Drop static-power related stuff

No one has used it for the last two and half years (since it was
introduced by commit c36cf071 (thermal: cpu_cooling: implement the
power cooling device API), get rid of it.

Acked-by: Eduardo Valentin <edubezval@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>