remove the N connection simulation from the Reno code

[marten-seemann]

Removing it from Cubic would be more work, since the hard coded congestion window sizes in the tests would have to be adjusted, and I don't know where to get the expected numbers from.

[codecov]

Codecov Report

Merging #2682 into master will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2682      +/-   ##
==========================================
- Coverage   86.77%   86.77%   -0.01%     
==========================================
  Files         124      124              
  Lines        9875     9869       -6     
==========================================
- Hits         8569     8563       -6     
  Misses        974      974              
  Partials      332      332              

Impacted Files	Coverage Δ
internal/congestion/cubic_sender.go	90.65% <100.00%> (-0.50%)	⬇️

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