dhcpv6: Remove CONFIRM message sending when connecting a link

The CONFIRM message is not sent any more as it does not make much
sense because we do not save address expiration time so we cannot
really know how long the saved address is valid anyway.

The reply to CONFIRM message does not send expiration times back
to us so we cannot get them from server either. Because of this
we need to start using SOLICITATION anyway when a link is
connected.

Thanks to Pasi Sjöholm for reporting this.