xen/events: remove unused functions

There are no users of xen_irq_from_pirq() and xen_set_irq_pending().

Remove those functions.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Signed-off-by: Juergen Gross <jgross@suse.com>