arc: 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/arc/kernel/reset.c

@@ -26,6 +26,3 @@ void machine_power_off(void)
 	/* FIXME ::  power off ??? */
 	machine_halt();
 }
-
-void (*pm_power_off) (void) = NULL;
-EXPORT_SYMBOL(pm_power_off);