test-remove the multithreaded optimization in the CSV reader

[vuule]

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• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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[res-life]

What's the perf impact of disabling multi-thread, do we have a number?

[vuule]

What's the perf impact of disabling multi-thread, do we have a number?

This optimization was added as part of #20996. I measured 30% improvement in our benchmarks when all columns are strings columns.

I think it's better to revert it and implement it more carefully/early in a future release, we are already deep in code freeze.