ACPI processor: Remove unneeded cpuidle_unregister_driver call

Since commit 46bcfad7 registering
and unregistering cpuidle is done in processor_idle.c.
Unregistering via:
acpi_bus_unregister_driver(&acpi_processor_driver)
   -> acpi_processor_remove()
      -> acpi_processor_power_exit()

Remove not needed cpuidle_unregister_driver() call from
acpi_processor_exit

Signed-off-by: Thomas Renninger <trenn@suse.de>
CC: Deepthi Dharwar <deepthi@linux.vnet.ibm.com>
Signed-off-by: Len Brown <len.brown@intel.com>