sagemathgh-38625: Remove outdated setting of sphinx_source_suffix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We remove a setting that is outdated since Sphinx 1.8 (!)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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URL: sagemath#38625
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee

src/sage_docbuild/conf.py

@@ -187,9 +187,6 @@ def sphinx_plot(graphics, **kwds):
 # Add any paths that contain templates here, relative to this directory.
 templates_path = [os.path.join(SAGE_DOC_SRC, 'common', 'templates'), 'templates']
 
-# The suffix of source filenames.
-source_suffix = '.rst'
-
 # The master toctree document.
 master_doc = 'index'