thread_id_pool.h: mark variable as [[maybe_unused]]

[wheatman]

mark a variable only used in an assert as [[maybe_unused]] so that non debug builds don't give warnings when compiling with -Wunused-but-set-variable

parlaylib/include/parlay/internal/../internal/../internal/thread_id_pool.h:50:12: warning: variable 'num_destroyed' set but not used [-Wunused-but-set-variable]
   50 |     size_t num_destroyed = 0;

[codecov-commenter]

Codecov Report

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

Comparison is base (f8e38c7) 96.84% compared to head (604e34f) 96.87%.

:exclamation: Current head 604e34f differs from pull request most recent head e4ff9a6. Consider uploading reports for the commit e4ff9a6 to get more accurate results

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #69 +/- ## ========================================== + Coverage 96.84% 96.87% +0.02% ========================================== Files 56 56 Lines 3708 3708 ========================================== + Hits 3591 3592 +1 + Misses 117 116 -1 ```

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