powerpc/smp: Fix generic_mach_cpu_die()

This is used by some "soft" hotplug implementations. I needs to
call idle_task_exit() when the CPU is going away, and we remove
the now no-longer needed set_cpu_online() and local_irq_enable()
which are handled by the return to start_secondary

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>