Style Guide prescriptions on ABI explicitness

[calebcartwright]

rust-lang/rfcs#3722 seems likely to land albeit for some future edition beyond 2024, and when it does I would propose we remove the existing ABI prescriptions from the Style Guide text

Formatters (at least rustfmt) do not necessarily have the relevant context to know whether formatting changes that involved the ABI would be semantics preserving, and the behavior that matched the "always be explicit" rule in the Style Guide did cause a couple of issues over the years (e.g. rust-lang/rustfmt#2908)

Once rust-lang/rfcs#3722 is enacted I think rustfmt should fallback to just maintaining ABI as it exists within the AST, and leave the consideration of whether an ABI has a default and/or is required as a lang item