ci: delete pr-differences-mutants workflow

[fdefelici]

Description

Deleting PR Differences Mutants workflow because no more used.
Reasoning about it can be found in the related issue #6136

Applicable issues

• fixes [CI] Delete unused PR Differences Mutants workflow  #6136

Additional info (benefits, drawbacks, caveats)

Checklist

• Test coverage for new or modified code paths
• Changelog is updated
• Required documentation changes (e.g., docs/rpc/openapi.yaml and rpc-endpoints.md for v2 endpoints, event-dispatcher.md for new events)
• New clarity functions have corresponding PR in clarity-benchmarking repo
• New integration test(s) added to bitcoin-tests.yml

[wileyj]

Since this workfow isn't being run, i think this is fine. Approving pending status checks passing
@ASuciuX - any comments or reason we should keep this workflow?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.70%. Comparing base (c125757) to head (c885d14).
Report is 274 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #6137      +/-   ##
===========================================
+ Coverage    82.89%   83.70%   +0.81%     
===========================================
  Files          542      542              
  Lines       390027   390027              
  Branches       323      323              
===========================================
+ Hits        323302   326463    +3161     
+ Misses       66717    63556    -3161     
  Partials         8        8              

see 56 files with indirect coverage changes

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