Remove `box_syntax` r? `@Nilstrieb` This removes the feature `box_syntax`, which allows the use of `box <expr>` to create a Box, and finalises removing use of the feature from the compiler. `box_patterns` (allowing the use of `box <pat>` in a pattern) is unaffected. It also removes `ast::ExprKind::Box` - the only way to create a 'box' expression now is with the rustc-internal `#[rustc_box]` attribute. As a temporary measure to help users move away, `box <expr>` now parses the inner expression, and emits a `MachineApplicable` lint to replace it with `Box::new` Closes #49733