Delete codecov.yml

[sobolevn]

I want to discuss whether or not we need it.

First of all, looks like it is ignored by codecov-action: https://github.com/codecov/codecov-action/blob/b049ab51f46ef6cd9c7ca52a856631a818969c7c/src/buildExec.ts#L19-L156

Other users report it as well: codecov/codecov-action#554

Secondly, it looks like our action works without it.
Run: sobolevn#2

So, I see two options:

1. Just delete it
2. Think of options we need to transfer to GitHub Actions with: arguments

[codecov]

Codecov Report

Merging #409 (209d7e8) into master (bbc8f7e) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #409   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines         1298      1298           
=========================================
  Hits          1298      1298           

[Mariatta]

Thanks! Let's try it!