Rollup merge of #160871 - WaffleLapkin:remove_rustc_reservation_impl, r=oli-obk,JonathanBrouwer

Remove `#[rustc_reservation_impl]`

Based on https://github.com/rust-lang/rust/pull/160705.

There was only ever one reservation impl and it got (/is being) removed in rust-lang/rust#160705. I don't think there is a reason to keep the machinery, given how specific and tricky the semantics of it are (see https://github.com/rust-lang/rust/issues/64631).

Closes https://github.com/rust-lang/rust/issues/64631.

cc @clarfonthey since you seem to have found another use for the attribute (https://github.com/rust-lang/rust/issues/64631#issuecomment-4633993125) (I haven't looked into it, so not sure if it might be worth keeping the attribute for that use case).

r? types