cpufreq: intel_pstate: Drop redundant locking from intel_pstate_driver_cleanup()

Remove the spinlock locking from intel_pstate_driver_cleanup() as it is
not necessary because no other code accessing all_cpu_data[] can run in
parallel with that function.

Had the locking been necessary, though, it would have been incorrect
because the lock in question is acquired from a hardirq handler and
it cannot be acquired from thread context without disabling interrupts.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>