simple-repository-api: remove partial TUF section

[woodruffw]

This is part of my efforts to make this page a bit easier to follow: this section is a bit out of place/out of step with the rest of the page's structure, especially now that the page is structured around distinct sections for the HTML and JSON index forms. It's also not immediately relevant to an index implementor/consumer for two reasons:

1. PEP 458 is (unfortunately IMO) still in limbo in terms of implementation and isn't marked as Final yet, which puts it a bit out of sync with the other PEPs that have been converted here (all of which are either Final or were added very shortly before becoming Final).
2. The current section doesn't say a ton about how index implementors or consumers should actually implement TUF; it links back to the PEP instead. This is out of line with the other living specs, which have been fully imported into the PyPUG specifications. I think a good move here would be doing the same for PEP 458, pending (1) above.

As best I can tell, this language for PEP 458 was imported in #1477 as part of the initial rewrite of this page. However, I'm not actually sure where the language itself came from 😅

CC @di

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📚 Documentation preview 📚: https://python-packaging-user-guide--1891.org.readthedocs.build/en/1891/

[di]

This makes sense to me given the status of the PEP relative to the rest of the page!