x86: delete unneeded declarations of disable_irq() and enable_irq()

These two declarations in arch/x86/include/asm/interrupt.h conflict
with ones in include/linux/compat.h, so x86 boards cannot include
<linux/compat.h>.

The comment /* arch/x86/lib/interrupts.c */ is bogus now, and we do
not see any definitions of disable_irq() and enable_irq() in there.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>