docs: remove obsolete warning

[rafaelsgirao]

Make sure these boxes are signed before submitting your Pull Request -- thank you.

• [X] I have read the contributing guide lines at https://redmine.openinfosecfoundation.org/projects/suricata/wiki/Contributing
• [X] I have signed the Open Information Security Foundation contribution agreement at https://suricata.io/about/contribution-agreement/
• [X] I have updated the user guide (in doc/userguide/) to reflect the changes made (if applicable)

Link to redmine ticket:

Contributing guidelines says a ticket is not necessary for minor and/or simple changes. Describe changes: Documentation dating to 2016(!) had a warning related to a bug in AF_PACKET, which has since been fixed (and the mentioned ticket closed).

[codecov[bot]]

Codecov Report

Merging #7323 (41302e7) into master (4bb0096) will decrease coverage by 0.01%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #7323      +/-   ##
==========================================
- Coverage   75.85%   75.83%   -0.02%     
==========================================
  Files         656      656              
  Lines      190065   190066       +1     
==========================================
- Hits       144171   144136      -35     
- Misses      45894    45930      +36     

Flag	Coverage Δ
fuzzcorpus	60.43% <ø> (-0.08%)	:arrow_down:
suricata-verify	51.59% <ø> (+0.03%)	:arrow_up:
unittests	61.01% <ø> (-0.01%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

[victorjulien]

Merged in #8815, thanks!