remove unused connectionStats counters from the Reno implementation

[marten-seemann]

No description provided.

[codecov]

Codecov Report

Merging #2683 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2683      +/-   ##
==========================================
- Coverage   86.74%   86.74%   -0.01%     
==========================================
  Files         124      124              
  Lines        9897     9893       -4     
==========================================
- Hits         8585     8581       -4     
  Misses        978      978              
  Partials      334      334              

Impacted Files	Coverage Δ
internal/congestion/cubic_sender.go	90.84% <ø> (-0.27%)	⬇️

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