Remove `#[staged_api]`

[brson]

The only purpose this serves is to make stability attributes modal, distinguishing between in-tree and out-of-tree use cases. #29952 though renamed #[deprecated] to #[rustc_deprecated], so afaik #[staged_api] does nothing useful.

[brson]

Fixing this should be a matter of removing the code that checks it in stability.rs, following the leads to other fallout in the compiler, removing the attribute from all the crates it's attached to.

[petrochenkov]

Hm, what about #[stable] and #[unstable]? They are guarded by #[staged_api] as well.
But they can probably be guarded by #[feature(staged_api)] itself instead.