Now that via_isa_set_irq() is unused it can be removed.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-Id: <20230531211043.41724-4-shentey@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>