xen/events: remove unnecessary call to bind_evtchn_to_cpu()

Since bind_evtchn_to_cpu() is always called after an event channel is
bound, there is no need to call it after closing an event channel.

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>