alpha: Remove pm_power_off

All users of pm_power_off() have been converted to new sys-off API.
Remove deprecated pm_power_off().

Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>

arch/alpha/kernel/process.c

@@ -43,12 +43,6 @@
 #include "proto.h"
 #include "pci_impl.h"
 
-/*
- * Power off function, if any
- */
-void (*pm_power_off)(void) = machine_power_off;
-EXPORT_SYMBOL(pm_power_off);
-
 #ifdef CONFIG_ALPHA_WTINT
 /*
  * Sleep the CPU.