Remove outdated information from the doc

[vmaurin]

aiokafka is not dependent anymore on kafka-python

Changes

Fixes #1032

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> (e.g. 588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the PR
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: Fix issue with non-ascii contents in doctest text files.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.02%. Comparing base (2f976d0) to head (0d91dbd).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1033   +/-   ##
=======================================
  Coverage   95.02%   95.02%           
=======================================
  Files         114      114           
  Lines       16970    16970           
  Branches     2771     2771           
=======================================
  Hits        16125    16125           
  Misses        497      497           
  Partials      348      348           

Flag	Coverage Δ
cext	91.94% <ø> (ø)
integration	94.66% <ø> (-0.02%)	⬇️
purepy	94.49% <ø> (ø)
unit	52.88% <ø> (ø)

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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[ods]

Thank you for the contribution! Added with other doc fixes in #1034