Remove some include directives for unused headers

[vittorioromeo]

Thanks a lot for making a contribution to SFML! 🙂

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• Has this change been discussed on the forum or in an issue before?
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Description

Self-explanatory.

Tasks

• Tested on Linux
• Tested on Windows
• Tested on macOS
• Tested on iOS
• Tested on Android

How to test this PR?

CI will be good enough.

[Bromeon]

Nice, thanks! 🙂

Out of curiosity, did you use tooling to detect unused headers?

[vittorioromeo]

Nice, thanks! 🙂

Out of curiosity, did you use tooling to detect unused headers?

Yes, but not in the way you think. I was actually benchmarking SFML's compilation time via ClangBuildAnalyzer. The biggest bottleneck, by far, is sf::Utf, followed by sf::String. I was surprised to see that sf::String was being included many times, then I did some manual searches and found unnecessary includes.

We could try out this next: https://github.com/include-what-you-use/include-what-you-use