ACPI: processor: unify arch_acpi_processor_cleanup_pdc

The x86 and ia64 implementations of the function in $subject are
exactly the same.

Also, since the arch-specific implementations of setting _PDC have
been completely hollowed out, remove the empty shells.

Cc: Tony Luck <tony.luck@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Len Brown <len.brown@intel.com>