bpf, docs: Remove deprecated xsk libbpf APIs description

Since xsk APIs has been removed from libbpf, let's clean up the
BPF docs simutaneously.

Signed-off-by: Pu Lehui <pulehui@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/bpf/20220708042736.669132-1-pulehui@huawei.com