sh: 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/sh/kernel/reboot.c

@@ -10,9 +10,6 @@
 #include <asm/tlbflush.h>
 #include <asm/traps.h>
 
-void (*pm_power_off)(void);
-EXPORT_SYMBOL(pm_power_off);
-
 static void watchdog_trigger_immediate(void)
 {
 	sh_wdt_write_cnt(0xFF);