src: mark unused private field as such

[targos]

Refs: #59828

[targos]

If there's a good reason to keep it, we should instead put a comment and disable compiler warnings.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.54%. Comparing base (d729bec) to head (93d241e).
⚠️ Report is 34 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #60802      +/-   ##
==========================================
- Coverage   88.55%   88.54%   -0.02%     
==========================================
  Files         703      703              
  Lines      208259   208259              
  Branches    40162    40170       +8     
==========================================
- Hits       184415   184393      -22     
- Misses      15844    15861      +17     
- Partials     8000     8005       +5     

Files with missing lines	Coverage Δ
src/node.h	95.91% <ø> (ø)

... and 49 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/70314/