x86/ioapic: Remove unused "IOAPIC_AUTO" define

Last use was removed more than 5 years ago, in:

   5ad274d4: ("x86/irq: Remove unused old IOAPIC irqdomain interfaces")

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200724114418.629021-2-mingo@kernel.org