The test fails randomly for unknown reason.
https://github.com/ruby/ruby/actions/runs/14121674932/job/39562945096?pr=12993
```
1) Error:
TestParallel::TestParallel#test_retry_workers:
Test::Unit::ProxyError: execution expired (start: 2025-03-28 04:04:10 +0000, end: 2025-03-28 04:05:50 +0000)
/home/runner/work/ruby/ruby/src/tool/test/testunit/test_parallel.rb:16:in 'TestParallel.timeout'
/home/runner/work/ruby/ruby/src/tool/test/testunit/test_parallel.rb:225:in 'TestParallel::TestParallel#test_retry_workers'
```
Maybe the workers do not respond "quit" request.
We no longer use the retry mechanism, so just remove the test.
fc26004660