Auto merge of #108471 - clubby789:unbox-the-syntax, r=Nilstrieb,est31

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

compiler/rustc_mir_transform/src/lib.rs

@@ -1,7 +1,6 @@
 #![allow(rustc::potential_query_instability)]
 #![feature(box_patterns)]
 #![feature(drain_filter)]
-#![feature(box_syntax)]
 #![feature(let_chains)]
 #![feature(map_try_insert)]
 #![feature(min_specialization)]