x86: Sanitize apb timer interrupt handling

Disable the interrupt in CPU_DEAD where it belongs. Remove the
open coded irq_desc manipulation.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@elte.hu>
Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>