[IA64] remove dead code: __cpu_{down,die} from !HOTPLUG_CPU

Neither __cpu_down() nor __cpu_die() are being referenced without
CONFIG_HOTPLUG_CPU.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>