Drop dead Connection.detach() and Connection.writer

[asvetlov]

Both methods were broken for more than 2 years.

[codecov-io]

Codecov Report

Merging #3358 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3358      +/-   ##
==========================================
- Coverage   98.06%   98.03%   -0.04%     
==========================================
  Files          44       44              
  Lines        8341     8334       -7     
  Branches     1370     1369       -1     
==========================================
- Hits         8180     8170      -10     
- Misses         66       68       +2     
- Partials       95       96       +1

Impacted Files	Coverage Δ
aiohttp/connector.py	98.16% <ø> (-0.03%)	⬇️
aiohttp/tcp_helpers.py	90% <0%> (-6.67%)	⬇️
aiohttp/client_reqrep.py	97.49% <0%> (-0.17%)	⬇️

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[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a [new issue] for related bugs.
If you feel like there's important points made in this discussion, please include those exceprts into that [new issue].
[new issue]: https://github.com/aio-libs/aiohttp/issues/new