Revert "Allow CI unit test set 1 to run multi-threaded"

[songwdfu]

Reverts #16099, since it causes some redundant unit tests to be ran across both unit test sets. #16269

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.21%. Comparing base (1a476de) to head (5064a70).
⚠️ Report is 555 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #16275      +/-   ##
============================================
+ Coverage     62.90%   63.21%   +0.31%     
+ Complexity     1386     1363      -23     
============================================
  Files          2867     2962      +95     
  Lines        163354   171497    +8143     
  Branches      24952    26273    +1321     
============================================
+ Hits         102755   108411    +5656     
- Misses        52847    54848    +2001     
- Partials       7752     8238     +486     

Flag	Coverage Δ
custom-integration1	100.00% <ø> (ø)
integration	100.00% <ø> (ø)
integration1	100.00% <ø> (ø)
integration2	0.00% <ø> (ø)
java-11	33.40% <ø> (-29.47%)	⬇️
java-21	63.20% <ø> (+0.38%)	⬆️
skip-bytebuffers-false	?
skip-bytebuffers-true	?
temurin	63.21% <ø> (+0.31%)	⬆️
unittests	63.21% <ø> (+0.31%)	⬆️
unittests1	56.30% <ø> (+0.48%)	⬆️
unittests2	33.40% <ø> (-0.17%)	⬇️

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