Remove support for post_pr_comment

[astrofrog]

Cunning plan for removing code!

[codecov-io]

Codecov Report

Merging #49 into master will decrease coverage by 2.37%.
The diff coverage is 90%.

@@            Coverage Diff             @@
##           master      #49      +/-   ##
==========================================
- Coverage   84.43%   82.05%   -2.38%     
==========================================
  Files          14       14              
  Lines         790      758      -32     
==========================================
- Hits          667      622      -45     
- Misses        123      136      +13

Impacted Files	Coverage Δ
baldrick/plugins/github_pull_requests.py	79.31% <90%> (-4.57%)	⬇️
baldrick/github/github_api.py	66.35% <0%> (-4.94%)	⬇️
baldrick/plugins/circleci_artifacts.py	100% <0%> (ø)	⬆️

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