thermal: cpu_cooling: Don't check is_cpufreq_valid()

Because get_cpu_frequency() has returned a valid frequency, it means that the
cpufreq policy is surely valid and so no point checking that again with
is_cpufreq_valid(). Get rid of the routine as well as there are no more users.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>