Remove GH actions pre-commit workflow

[dstansby]

Since pre-commit.ci runs the pre-commit checks on pull requests/pushes to main, I don't think there's a need to duplicate it on GitHub actions?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.70%. Comparing base (56f72b0) to head (a142b96).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #398      +/-   ##
==========================================
+ Coverage   85.44%   85.70%   +0.25%     
==========================================
  Files          13       13              
  Lines        1539     1539              
==========================================
+ Hits         1315     1319       +4     
+ Misses        224      220       -4     

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