include/hw/or-irq.h: Drop unused in_irqs field

The struct OrIRQState has an unused member field in_irqs.
This is a legacy of earlier versions of the patch; the
code that used it was dropped from the final version of
the code that went into master, but we forgot to delete
the no-longer-used struct field. Do so now.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>