polish: Remove redundant attrs on `unchecked_ref_cast_mut`

[dtolnay]

• https://github.com/dtolnay/ref-cast/pull/38 fixes the unused_unsafe lint
• https://github.com/dtolnay/ref-cast/pull/39 makes ref_cast_custom functions inline by default

[ConnorGray]

Thanks for contributing these changes, David. ref-cast has been a powerful tool in several projects I've worked on.