m68k: 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/m68k/kernel/process.c

@@ -71,9 +71,6 @@ void machine_power_off(void)
 	for (;;);
 }
 
-void (*pm_power_off)(void);
-EXPORT_SYMBOL(pm_power_off);
-
 void show_regs(struct pt_regs * regs)
 {
 	pr_info("Format %02x  Vector: %04x  PC: %08lx  Status: %04x    %s\n",