selftests: icmp_redirect: remove from checking for IPv6 route get

If the kernel doesn't enable option CONFIG_IPV6_SUBTREES, the RTA_SRC
info will not be exported to userspace in rt6_fill_node(). And ip cmd will
not print "from ::" to the route output. So remove this check.

Fixes: ec810535

 ("selftests: Add redirect tests")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>