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 <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Acked-by: Song Liu <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

Documentation/bpf/libbpf/libbpf_naming_convention.rst

···
 new function or type is added to keep libbpf API clean and consistent.
 
 All types and functions provided by libbpf API should have one of the
-following prefixes: ``bpf_``, ``btf_``, ``libbpf_``, ``xsk_``,
-``btf_dump_``, ``ring_buffer_``, ``perf_buffer_``.
+following prefixes: ``bpf_``, ``btf_``, ``libbpf_``, ``btf_dump_``,
+``ring_buffer_``, ``perf_buffer_``.
 
 System call wrappers
 --------------------
···
 Auxiliary functions and types that don't fit well in any of categories
 described above should have ``libbpf_`` prefix, e.g.
 ``libbpf_get_error`` or ``libbpf_prog_type_by_name``.
-
-AF_XDP functions
--------------------
-
-AF_XDP functions should have an ``xsk_`` prefix, e.g.
-``xsk_umem__get_data`` or ``xsk_umem__create``. The interface consists
-of both low-level ring access functions and high-level configuration
-functions. These can be mixed and matched. Note that these functions
-are not reentrant for performance reasons.
 
 ABI
 ---