bpf, doc: Fix some invalid links in bpf_devel_QA.rst

bpf, doc: Fix some invalid links in bpf_devel_QA.rst

There exist some errors "404 Not Found" when I click the link
of "MAINTAINERS" [1], "samples/bpf/" [2] and "selftests" [3]
in the documentation "HOWTO interact with BPF subsystem" [4].

As Alexei Starovoitov suggested, just remove "MAINTAINERS" and
"samples/bpf/" links and use correct link of "selftests".

  [1] https://www.kernel.org/doc/html/MAINTAINERS
  [2] https://www.kernel.org/doc/html/samples/bpf/
  [3] https://www.kernel.org/doc/html/tools/testing/selftests/bpf/
  [4] https://www.kernel.org/doc/html/latest/bpf/bpf_devel_QA.html

Fixes: 542228384888 ("bpf, doc: convert bpf_devel_QA.rst to use RST formatting")
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Link: https://lore.kernel.org/bpf/1619062560-30483-1-git-send-email-yangtiezhu@loongson.cn