Remove uses of box_syntax in rustc and tools

compiler/rustc_error_codes/src/error_codes/E0010.md

@@ -5,7 +5,5 @@ the heap at runtime, and therefore cannot be done at compile time.
 Erroneous code example:
 
 ```compile_fail,E0010
-#![feature(box_syntax)]
-
-const CON : Box<i32> = box 0;
+const CON : Vec<i32> = vec![1, 2, 3];
 ```