selftests/bpf: remove xdp_synproxy IP_DF check

In real world production websites, the IP_DF flag
is not always set for each packet from these websites.
the IP_DF flag check breaks Internet connection to
these websites for home based firewall like BPFire
when XDP synproxy program is attached to firewall
Internet facing side interface. see [0]

[0] https://github.com/vincentmli/BPFire/issues/59



Signed-off-by: Vincent Li <vincent.mc.li@gmail.com>
Link: https://lore.kernel.org/r/20241025031952.1351150-1-vincent.mc.li@gmail.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>