Remove PDF images

[StanFromIreland]

With #145480, these will be automatically converted if necessary.

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📚 Documentation preview 📚: https://cpython-previews--145977.org.readthedocs.build/

[StanFromIreland]

CC @m-aciek

[StanFromIreland]

EPUB is failing, but it has always used the SVG? I'll try figure it out.

Edit: It seems .. only:: html is in fact, not only HTML.

[m-aciek]

I suggest to use builder_ prefix for readability disambiguity.

• https://www.sphinx-doc.org/en/master/usage/restructuredtext/directives.html#directive-only
• https://www.sphinx-doc.org/en/master/usage/builders/index.html#sphinx.builders.html.StandaloneHTMLBuilder

[StanFromIreland]

Good catch, thanks @m-aciek!