src: remove unused private field

[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 (35cece7).
⚠️ Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #60802      +/-   ##
==========================================
- Coverage   88.55%   88.54%   -0.01%     
==========================================
  Files         703      703              
  Lines      208259   208259              
  Branches    40162    40158       -4     
==========================================
- Hits       184415   184400      -15     
- Misses      15844    15887      +43     
+ Partials     8000     7972      -28     

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

... and 44 files with indirect coverage changes

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[legendecas]

This structure is a public API and changing the private fields will change the size of the struct and cause ABI incompatibility.

This field was named as reserved_.

This should be a semver-major change.

[targos]

I'm ok to keep it if it's reserved for something. Do you have a suggestion for an explaining comment?

[addaleax]

Calling it void* reserved_; should be fine without a comment tbh -- this is a pretty common pattern in C/C++ APIs for ABI forward compatibility.

[legendecas]

I think renaming this field to reserved_ could be good. We can also remove this in a semver-major change.