gh-38625: Remove outdated setting of `sphinx_source_suffix`
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The docbuild shows repeated messages
```
Converting `source_suffix = '.rst'` to `source_suffix = {'.rst':
'restructuredtext'}`
```
We remove a setting that is outdated since Sphinx 1.8 (!)
https://www.sphinx-doc.org/en/master/usage/configuration.html#confval-
source_suffix
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### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/38625
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee
