refactor: Remove non-functional record-flattening capability

[ReubenFrankel]

As per https://meltano.slack.com/archives/C06A1LKFAAC/p1725964942643019, record-flatting appears to be unused - all current flattening logic pertains to schema-flattening only.

Follow-on PRs:

• refactor: Remove record-flattening capability in favour of schema-flattening meltano#8762
• chore: Remove record-flattening capability in favour of schema-flattening hub#1839
• chore: Remove record-flattening capability hub-utils#154

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📚 Documentation preview 📚: https://meltano-sdk--2666.org.readthedocs.build/en/2666/

[codspeed-hq]

CodSpeed Performance Report

Merging #2666 will not alter performance

_{Comparing ReubenFrankel:chore/record-flattening-unused (899e41f) with main (998083a)}

Summary

✅ 6 untouched benchmarks

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.86%. Comparing base (998083a) to head (899e41f).
Report is 113 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2666      +/-   ##
==========================================
- Coverage   89.86%   89.86%   -0.01%     
==========================================
  Files          58       58              
  Lines        4886     4885       -1     
  Branches      960      960              
==========================================
- Hits         4391     4390       -1     
  Misses        344      344              
  Partials      151      151              

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[edgarrmondragon]

I wanted to suggest going though a deprecation period but absolutely nobody is using this 😅

[edgarrmondragon]

the griffe failure is expected cause we're removing something from a public API, ok to ignore

[ReubenFrankel]

I'll ready those other PRs now. 🙂