Clean up some code related to #[no_move].

The patch makes RootCollection a bit safer by making the StackRootTLS hold
it in place.

The use of no_move in CodeGenRust was leftover from when roots couldn't
be moved.

components/script/dom/bindings/js.rs

@@ -453,7 +453,6 @@ impl<T: Reflectable> OptionalRootedReference<T> for Option<Option<Root<T>>> {
 ///
 /// See also [*Exact Stack Rooting - Storing a GCPointer on the CStack*]
 /// (https://developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey/Internals/GC/Exact_Stack_Rooting).
-#[no_move]
 pub struct RootCollection {
     roots: UnsafeCell<Vec<*const Reflector>>,
 }