cpufreq: Remove unused flag CPUFREQ_PM_NO_WARN

This flag is set by one of the drivers but it isn't used in the code
otherwise. Remove the unused flag and update the driver.

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