bgpd: fix radv interface disabled when bgp instance removed

bgpd: fix radv interface disabled when bgp instance removed

If a peer uses radv for an interface, and bgp instance is removed,
then the radv service is not disabled on the interface.

Fix this by doing the same at BGP unconfiguration. Like it has been
done when a peer is unconfigured, call the radv unregistration before
deleting the peer.

Fixes: b3a3290e2303 ("bgpd: turn off RAs when numbered peers are deleted")
Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>
Signed-off-by: Dmytro Shytyi <dmytro.shytyi@6wind.com>