[IPV6]: Remove useless code from fib6_del_route().

There are useless codes in fib6_del_route(). The following patch has
been tested, every thing looks fine, as usual.

Signed-off-by: Gui Jianfeng <guijianfeng@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>