ipv6: remove parameter rt from fib6_prune_clones()

the parameter rt will be assigned to c.arg in function fib6_clean_tree(),
but function fib6_prune_clone() doesn't use c.arg, so we can remove it
safely.

Signed-off-by: Duan Jiong <duanj.fnst@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>