wireguard: selftests: remove old conntrack kconfig value

commit acf2492b

 upstream.

On recent kernels, this config symbol is no longer used.

Reported-by: Rui Salvaterra <rsalvaterra@gmail.com>
Fixes: e7096c13

 ("net: WireGuard secure network tunnel")
Cc: stable@vger.kernel.org
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>