Rollup merge of #108958 - clubby789:unbox-the-hir, r=compiler-errors

Remove box expressions from HIR

After #108516, `#[rustc_box]` is used at HIR->THIR lowering and this is no longer emitted, so it can be removed.

This is based on top of #108471 to help with conflicts, so 43490488ccacd1a822e9c621f5ed6fca99959a0b is the only relevant commit (sorry for all the duplicated pings!)

````@rustbot```` label +S-blocked

compiler/rustc_mir_build/src/thir/cx/expr.rs

@@ -780,7 +780,6 @@ impl<'tcx> Cx<'tcx> {
             hir::ExprKind::DropTemps(ref source) => {
                 ExprKind::Use { source: self.mirror_expr(source) }
             }
-            hir::ExprKind::Box(ref value) => ExprKind::Box { value: self.mirror_expr(value) },
             hir::ExprKind::Array(ref fields) => {
                 ExprKind::Array { fields: self.mirror_exprs(fields) }
             }