Cleanup after kafka-python merge

[ods]

Changes

Follow-up to merge of python-kafka, removes dead code.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (8814e7b) 84.25% compared to head (fdba764) 94.88%.

Files	Patch %	Lines
aiokafka/protocol/message.py	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #947       +/-   ##
===========================================
+ Coverage   84.25%   94.88%   +10.63%     
===========================================
  Files          87      106       +19     
  Lines       10040    16136     +6096     
  Branches        0     2592     +2592     
===========================================
+ Hits         8459    15311     +6852     
+ Misses       1581      551     -1030     
- Partials        0      274      +274     

Flag	Coverage Δ
cext	91.57% <0.00%> (+12.26%)	⬆️
integration	94.55% <0.00%> (+10.32%)	⬆️
purepy	94.41% <0.00%> (+10.41%)	⬆️
unit	45.19% <0.00%> (-6.25%)	⬇️

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