selftests, bpf: Do not yet switch to new libbpf XDP APIs

Revert commit 54435652 ("selftests/bpf: switch to new libbpf XDP APIs")
for now given this will heavily conflict with 4b27480d ("bpf/selftests:
convert xdp_link test to ASSERT_* macros") upon merge. Andrii agreed to redo
the conversion cleanly after trees merged.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>