Remove randomness_in_documentation_underscore_downloads_generated_by_sphinx

Fixed in buster:

sphinx (1.8.4-1) unstable; urgency=medium
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  * Ensure the graphviz filenames are reproducible (closes: #921513).
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 -- Dmitry Shachnev <mitya57@debian.org>  Sat, 16 Feb 2019 16:55:01 +0300