[MIPS] Revert "mips: add pm_power_off"

pm_power_off duplicates the functionality of _machine_restart.
    
This reverts b142159fa5 commit.

arch/mips/kernel/reset.c

@@ -12,9 +12,6 @@
 #include <linux/reboot.h>
 #include <asm/reboot.h>
 
-void (*pm_power_off)(void);
-EXPORT_SYMBOL(pm_power_off);
-
 /*
  * Urgs ...  Too many MIPS machines to handle this in a generic way.
  * So handle all using function pointers to machine specific
@@ -36,9 +33,6 @@ void machine_halt(void)
 
 void machine_power_off(void)
 {
-	if (pm_power_off)
-		pm_power_off();
-
 	_machine_power_off();
 }