Revert "gh-135410: use a critical section around StringIO.__next__ (#135412)"

[ZeroIntensity]

This reverts commit e6c3039.

cc @vstinner @kumaraditya303 @corona10

• Issue: Segfault from calling StringIO methods in threads on free-threading debug build #135410

[kumaraditya303]

Well I don't feel that reverting it is necessary, the docs and benchmarks can be done on top but if others feel it is neccessary I am fine with it.

[corona10]

Let’s revert this change and revisit what better approaches we can take.
If we can’t guarantee that the Python version is thread-safe, then we should document that StringIO is not thread-safe. it also means we don’t need to guarantee thread safety in the C implementation either.
However, that doesn’t mean we should allow segmentation faults. (thread-safe and segmentation fault are fully different issues, at least we should guarantee thread-unsafe with memory-safe implementation.)

[miss-islington-app]

Thanks @ZeroIntensity for the PR, and @corona10 for merging it 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖

[bedevere-app]

GH-135449 is a backport of this pull request to the 3.14 branch.