x86, legacy_irq: Remove left over nr_legacy_irqs

nr_legacy_irqs and its ilk have moved to legacy_pic.

-v2: there is one in ioapic_.c

Singed-off-by: Yinghai Lu <yinghai@kernel.org>
LKML-Reference: <4B84AAC4.2020204@kernel.org>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>