Remove feature(box_syntax) from unused allocation list test

tests/ui/lint/unused/unused-allocation.rs

@@ -1,7 +1,7 @@
-#![feature(box_syntax)]
+#![feature(rustc_attrs, stmt_expr_attributes)]
 #![deny(unused_allocation)]
 
 fn main() {
-    _ = (box [1]).len(); //~ error: unnecessary allocation, use `&` instead
+    _ = (#[rustc_box] Box::new([1])).len(); //~ error: unnecessary allocation, use `&` instead
     _ = Box::new([1]).len(); //~ error: unnecessary allocation, use `&` instead
 }

tests/ui/lint/unused/unused-allocation.stderr

@@ -1,8 +1,8 @@
 error: unnecessary allocation, use `&` instead
   --> $DIR/unused-allocation.rs:5:9
    |
-LL |     _ = (box [1]).len();
-   |         ^^^^^^^^^
+LL |     _ = (#[rustc_box] Box::new([1])).len();
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
 note: the lint level is defined here
   --> $DIR/unused-allocation.rs:2:9