src: remove redundant handle_ field in ffi

[addaleax]

This was just a copy of the immediately preceding
handle_ field in uv_lib_t.

[nodejs-github-bot]

Review requested:

• @nodejs/ffi

[codecov]

Codecov Report

❌ Patch coverage is 64.28571% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.33%. Comparing base (17e4196) to head (2573814).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node_ffi.cc	64.28%	0 Missing and 5 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #63665   +/-   ##
=======================================
  Coverage   90.33%   90.33%           
=======================================
  Files         732      732           
  Lines      236454   236455    +1     
  Branches    44540    44527   -13     
=======================================
+ Hits       213589   213612   +23     
+ Misses      14581    14543   -38     
- Partials     8284     8300   +16     

Files with missing lines	Coverage Δ
src/node_ffi.h	61.53% <ø> (ø)
src/node_ffi.cc	69.95% <64.28%> (-0.10%)	⬇️

... and 32 files with indirect coverage changes

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