MIPS: 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/mips/kernel/reset.c

@@ -25,9 +25,6 @@
  */
 void (*_machine_restart)(char *command);
 void (*_machine_halt)(void);
-void (*pm_power_off)(void);
-
-EXPORT_SYMBOL(pm_power_off);
 
 static void machine_hang(void)
 {