[DOC] Remove unneeded LaTeX code from latex_elements['preamble']

[jfbu]

There is absolutely no reason in cpython Doc/conf.py that I can see to get all of the latex documents giving PDF via the xelatex tool to execute in their preambles:

\let\Verbatim=\OriginalVerbatim
\let\endVerbatim=\endOriginalVerbatim

These two lines do nothing and simply go full circle to end up letting \Verbatim be ... \Verbatim. From file sphinxlatexliterals.sty (which gets executed earlier than above preamble additions)

% Prior to Sphinx 1.5, \Verbatim and \endVerbatim were modified by Sphinx.
% The aliases defined here are used in sphinxVerbatim environment and can
% serve as hook-points with no need to modify \Verbatim itself.
\let\OriginalVerbatim   \Verbatim
\let\endOriginalVerbatim\endVerbatim

I don't know why this was added to conf.py at some point, but it definitely is not to be done now, and may confuse people who take big project such as cpython as models or comparison for using Sphinx.

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📚 Documentation preview 📚: https://cpython-previews--124630.org.readthedocs.build/

[python-cla-bot]

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[github-actions]

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

[serhiy-storchaka]

LGTM. 👍