xen/xenbus: remove unused xenbus_bind_evtchn()

xenbus_bind_evtchn() has no callers so remove it.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>