selftests: bpf: remove duplicate .flags initialization in ctx_skb.c

verifier/ctx_skb.c:708:11: warning: initializer overrides prior initialization of this subobject [-Winitializer-overrides]
        .flags = F_NEEDS_EFFICIENT_UNALIGNED_ACCESS,
                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>