This will allow us to check if all values are used

…onur-ozkan

Remove unused bootstrap config option

I tried to destructure a few of the TOML config structs to find any unused fields. I found that `Rust::run_dysmutil` field is unused. This PR uses destructuring of the `Rust` struct, to find similar unused fields in the future, and also removes the unused field.

I also found one more unused field (`Dist::gpg_password_file`), it doesn't seem to be used anywhere. If you like this PR, I'll send another one that uses destructuring for all interesting TOML structs and removes that unused field.

r? `@onur-ozkan`

Use immediate_backend_type when reading from a const alloc

Fixes rust-lang#118047

r? `@nikic`