Refactor web_protocol

[asvetlov]

• Drop dead code
• Make sanity check working in release mode

[codecov-io]

Codecov Report

Merging #3540 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3540      +/-   ##
==========================================
+ Coverage   97.88%   97.94%   +0.06%     
==========================================
  Files          43       43              
  Lines        8585     8608      +23     
  Branches     1388     1400      +12     
==========================================
+ Hits         8403     8431      +28     
+ Misses         75       74       -1     
+ Partials      107      103       -4

Impacted Files	Coverage Δ
aiohttp/web_exceptions.py	100% <ø> (ø)	⬆️
aiohttp/web_protocol.py	92.64% <100%> (+0.94%)	⬆️
aiohttp/client_reqrep.py	97.84% <0%> (+0.41%)	⬆️

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