src: remove NonCopyableMaybe

[tniessen]

I added this class in 823d86c47ce15fba8875fcebd412593b02aab362 in 2018 when we did not yet use std::optional. The last uses were removed in 5b9bf39b47afcf26aaad810eec2ba8bdff904d4c, so remove it.

Refs: https://github.com/nodejs/node/pull/24234 Refs: https://github.com/nodejs/node/pull/55368

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/66592/

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.12%. Comparing base (5fb879c) to head (4ce702a). Report is 16 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #58168      +/-   ##
==========================================
- Coverage   90.13%   90.12%   -0.01%     
==========================================
  Files         630      630              
  Lines      186611   186611              
  Branches    36631    36635       +4     
==========================================
- Hits       168204   168188      -16     
- Misses      11192    11194       +2     
- Partials     7215     7229      +14     

Files with missing lines	Coverage Δ
src/util.h	91.22% <ø> (ø)

... and 25 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/66604/

[nodejs-github-bot]

Landed in b0f2aa997349578704f3737fafd5ca3b257710f1