cpufreq: Drop unnecessary cpus locking from store()

This change was introduced long back by commit 4f750c93 ("cpufreq:
Synchronize the cpufreq store_*() routines with CPU hotplug").

Since then, both cpufreq and hotplug core have been reworked and have
much better locking in place. The race mentioned in commit 4f750c93
isn't possible anymore.

Drop the unnecessary locking.

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