selftests/bpf/test_offload.py: Remove check for program load flags match

Since we just removed the xdp_attachment_flags_ok() callback, also remove
the check for it in test_offload.py, and replace it with a test for the new
ambiguity-avoid check when multiple programs are loaded.

Fixes: 7f0a8382 ("bpf, xdp: Maintain info on attached XDP BPF programs in net_device")
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/bpf/160752225858.110217.13036901876869496246.stgit@toke.dk