xen/evtchn: Remove unused function declaration xen_set_affinity_evtchn()

Commit 67473b81 ("xen/events: Remove disfunct affinity spreading")
leave this unused declaration.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Rahul Singh <rahul.singh@arm.com>
Link: https://lore.kernel.org/r/20230801145413.40684-1-yuehaibing@huawei.com


Signed-off-by: Juergen Gross <jgross@suse.com>