Remove support for GitHub distributor

[AlCutter]

This PR removes support for sending checkpoints to the GitHub Actions based distributor.

This was an early experiment which has been superseded by the new REST distributor.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (feb984b) 48.37% compared to head (117ef74) 51.27%.
Report is 1 commits behind head on main.

❗ Current head 117ef74 differs from pull request most recent head 63d12bc. Consider uploading reports for the commit 63d12bc to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #131      +/-   ##
==========================================
+ Coverage   48.37%   51.27%   +2.90%     
==========================================
  Files          11       11              
  Lines         953      899      -54     
==========================================
  Hits          461      461              
+ Misses        424      370      -54     
  Partials       68       68              

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