chore: remove contrib mutation testing

[fdefelici]

Description

This PR is a follow-up to #6254 (related discussion)
It removes the unused mutation-testing script from the contrib/ folder, along with the associated documentation.

Applicable issues

• fixes chore: remove mutation-testing from contrib #6275

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

[fdefelici]

@wileyj Since mutation-testing is being completely removed, I could also include in this PR:

• Removing all #[cfg_attr(test, mutation::skip)] tags from the codebase
• Dropping the mutant crate dependency
• Closing all GitHub issues (referenced in the codebase) related to mutation-testing

What do you think?

[fdefelici]

@wileyj Since mutation-testing is being completely removed, I could also include in this PR:

• Removing all #[cfg_attr(test, mutation::skip)] tags from the codebase
• Dropping the mutant crate dependency
• Closing all GitHub issues (referenced in the codebase) related to mutation-testing

What do you think?

As shared during the nakasync, we’ll just update the documentation and remove the script from the contrib/ folder, while keeping the rest for potential future use.

[wileyj]

lgtm

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.50%. Comparing base (49727a3) to head (7ad4042).
⚠️ Report is 28 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #6512      +/-   ##
===========================================
- Coverage    80.03%   79.50%   -0.53%     
===========================================
  Files          565      565              
  Lines       346446   346446              
===========================================
- Hits        277282   275458    -1824     
- Misses       69164    70988    +1824     

see 65 files with indirect coverage changes

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