Yeet owning_ref

Turns out
- `owning_ref` is unsound due to `Box` aliasing stuff
- `rustc` doesn't need 99% of the `owning_ref` API
- `rustc` can use a far simpler abstraction that is `OwnedSlice`

compiler/rustc_data_structures/src/lib.rs

@@ -61,7 +61,6 @@ pub mod intern;
 pub mod jobserver;
 pub mod macros;
 pub mod obligation_forest;
-pub mod owning_ref;
 pub mod sip128;
 pub mod small_c_str;
 pub mod small_str;