selftests: net: fix nexthop warning cleanup double ip typo

I made a stupid typo when adding the nexthop route warning selftest and
added both $IP and ip after it (double ip) on the cleanup path. The
error doesn't show up when running the test, but obviously it doesn't
cleanup properly after it.

Fixes: 392baa33

 ("selftests: net: add delete nexthop route warning test")
Signed-off-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>