x86: fix up a comment in ack_APIC_irq()

 Adjust a comment in ack_APIC_irq() according to the recent removal of
CONFIG_X86_GOOD_APIC.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>