Remove use of soon-to-be-deprecated unstable feature

rust-lang/rust#47336 (comment)

components/script/dom/bindings/root.rs

@@ -383,7 +383,7 @@ where
     #[allow(unrooted_must_root)]
     pub unsafe fn from_box(value: Box<T>) -> Self {
         Self {
-            ptr: Box::into_raw_non_null(value),
+            ptr: Box::leak(value).into(),
         }
     }
 }

components/script/lib.rs

@@ -2,7 +2,6 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
-#![feature(box_into_raw_non_null)]
 #![feature(const_fn)]
 #![feature(const_transmute)]
 #![feature(core_intrinsics)]