Remove KeyDecoder[Repo] instance

[fthomas]

This is unused since #1220.

See #2281 (comment).

@alejandrohdezma I'm removing the instance here to keep the diff of #2281 focused on branch selection.

[codecov]

Codecov Report

Merging #2291 (33135b9) into master (49fe121) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2291   +/-   ##
=======================================
  Coverage   77.86%   77.86%           
=======================================
  Files         135      135           
  Lines        2290     2286    -4     
  Branches       60       58    -2     
=======================================
- Hits         1783     1780    -3     
+ Misses        507      506    -1     

Impacted Files	Coverage Δ
...in/scala/org/scalasteward/core/vcs/data/Repo.scala	100.00% <ø> (+16.66%)	⬆️

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