ACPI: cleanup acpi.h

Two cleanups to <linux/acpi.h>:
* Stop defining acpi_mp_config, it isn't used anywhere.
* Discard nested "#ifdef CONFIG_ACPI", they are useless and
  error-prone.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Len Brown <len.brown@intel.com>