Rollup merge of #104383 - WaffleLapkin:rustc_undiagnostic_item, r=compiler-errors

Rollup merge of #104383 - WaffleLapkin:rustc_undiagnostic_item, r=compiler-errors

Remove unused symbols and diagnostic items

As the title suggests, this removes unused symbols from `sym::` and `#[rustc_diagnostic_item]` annotations that weren't mentioned anywhere.

Originally I tried to use grep, to find symbols and item names that are never mentioned via `sym::name`, however this produced a lot of false positives (?), for example clippy matching on `Symbol::as_str` or macros "implicitly" adding `sym::`. I ended up fixing all these false positives (?) by hand, but tbh I'm not sure if it was worth it...