Remove box_syntax #14504

rust-lang/rust#108471 removed box_syntax (don't be confused with box_patterns, which is a distinct feature and unaffected). rustc now emits a temporary lint to replace them with Box::new() to facilitate the migration. We should follow the removal, but I think it's best to do so only after the migration seems to be mostly done and the lint is removed.

Filing this issue so we won't forget and so we can also discuss when to remove if anyone has opinion.