Drop encoding param

[asvetlov]

It was deprecated in aiohttp 2.0 and actually ignored

[codecov-io]

Codecov Report

Merging #2606 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2606   +/-   ##
=======================================
  Coverage   97.87%   97.87%           
=======================================
  Files          38       38           
  Lines        7304     7304           
  Branches     1263     1263           
=======================================
  Hits         7149     7149           
  Misses         51       51           
  Partials      104      104

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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