selftests/bpf: Retire bpf_tcp_helpers.h

The previous patches have consolidated the tests to use
bpf_tracing_net.h (i.e. vmlinux.h) instead of bpf_tcp_helpers.h.

This patch can finally retire the bpf_tcp_helpers.h from
the repository.

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20240509175026.3423614-11-martin.lau@linux.dev


Signed-off-by: Alexei Starovoitov <ast@kernel.org>