hexagon: 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/hexagon/kernel/reset.c

@@ -19,6 +19,3 @@ void machine_halt(void)
 void machine_restart(char *cmd)
 {
 }
-
-void (*pm_power_off)(void) = NULL;
-EXPORT_SYMBOL(pm_power_off);