Remove outdated optimization documentation

[kronion]

Description

Per #5873 and #4693 (comment) it appears that librabbitmq is no longer faster or recommended vs pyamqp. Some references to librabbitmq were already removed from the documentation, but this PR removes another that was missed.

This is purely a documentation change.

[codecov]

Codecov Report

Merging #6933 (f24d023) into master (e726978) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #6933   +/-   ##
=======================================
  Coverage   89.22%   89.22%           
=======================================
  Files         138      138           
  Lines       16711    16711           
  Branches     2116     2116           
=======================================
  Hits        14911    14911           
  Misses       1577     1577           
  Partials      223      223           

Flag	Coverage Δ
unittests	89.22% <ø> (ø)

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

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e726978...f24d023. Read the comment docs.