selftests/bpf: remove test_tcp_check_syncookie

Now that btf_skc_cls_ingress has the same coverage as
test_tcp_check_syncookie, remove the second one and keep the first one
as it is integrated in test_progs

Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore@bootlin.com>
Link: https://lore.kernel.org/r/20241020-syncookie-v2-6-2db240225fed@bootlin.com


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