ACPI: Do not prepare for hibernation in acpi_shutdown

Since we are now explicitly calling hibernation_ops->prepare() before
hibernation_ops->enter() in hibernation_platform_enter() (defined in
kernel/power/disk.c), ACPI should not call acpi_sleep_prepare(ACPI_STATE_S4)
from acpi_shutdown().

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Pavel Machek <pavel@ucw.cz>
Cc: Len Brown <lenb@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>