x86: I/O APIC: remove an IRQ2-mask hack

Now that IRQ2 is never made available to the I/O APIC, there is no need
to special-case it and mask as a workaround for broken systems.  Actually,
because of the former, mask_IO_APIC_irq(2) is a no-op already.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Cc: Matthew Garrett <mjg59@srcf.ucam.org>
Cc: Andreas Herrmann <andreas.herrmann3@amd.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Ingo Molnar <mingo@elte.hu>