Remove usage of unstable box syntax, except in the script crate

… because there’s a lot of it,
and script still uses any other unstable features anyway.

components/script_layout_interface/lib.rs

@@ -7,7 +7,6 @@
 //! to depend on script.
 
 #![deny(unsafe_code)]
-#![feature(box_syntax)]
 #![feature(nonzero)]
 
 extern crate app_units;