PM / Hibernate: Remove arch_prepare_suspend()

All architectures supporting hibernation define
arch_prepare_suspend() as an empty function, so remove it.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>

arch/powerpc/kernel/swsusp.c

@@ -10,7 +10,6 @@
  */
 
 #include <linux/sched.h>
-#include <asm/suspend.h>
 #include <asm/system.h>
 #include <asm/current.h>
 #include <asm/mmu_context.h>