Remove mention of 'Unicode' strings in documentation

[Viicos]

This was written when Python 2 was still relevant, so I think we can remove the explicit 'Unicode' mention here.

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📚 Documentation preview 📚: https://cpython-previews--136215.org.readthedocs.build/

[github-actions]

This PR is stale because it has been open for 30 days with no activity.

[serhiy-storchaka]

But this was written only for Python 3. Other programming languages can have 8-bit strings, and other Python libraries can serialize to bytes objects. So I think it will not harm to highlight that the json module works with Unicode strings.

[Viicos]

But this was written only for Python 3.

According to the blame, it was for 2.7? But fair enough, I think we can leave it as is.

[serhiy-storchaka]

Note that it says "Unicode strings", but refers to str, which is not an Unicode string in Python 2.
The latest version does not contain such text. The logs for Doc/library/json.rst don't contain such change. The real change was in f3e0a69 which does not contain such passage.