Remove the `overlapping_marker_traits` feature

Remove the `overlapping_marker_traits` feature

See #29864

This has been replaced by `#[feature(marker_trait_attr)]`

A few notes:

* Due to PR #68057 not yet being in the bootstrap compiler, it's
  necessary to continue using `#![feature(overlapping_marker_traits)]`
  under `#[cfg(bootstrap)]` to work around type inference issues.
* I've updated tests that used `overlapping_marker_traits` to now use
  `marker_trait_attr` where applicable

The test `src/test/ui/overlap-marker-trait.rs` doesn't make any sense
now that `overlapping_marker_traits`, so I removed it.

The test `src/test/ui/traits/overlap-permitted-for-marker-traits-neg.rs`
now fails, since it's no longer possible to have multiple overlapping
negative impls of `Send`. I believe that this is the behavior we want
(assuming that `Send` is not going to become a `#[marker]` trait, so I
renamed the test to `overlap-permitted-for-marker-traits-neg`