ipv6: remove a useless pr_info() in addrconf_gre_config()

This is debug info, should at least be pr_debug(), but given
that this code is in upstream for two years, there is no
need to keep this debugging printk any more, so just remove it.

Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>