Remove unrooted_must_root annotation from unions (fixes #2661).

The unsafety was fixed as part of the SpiderMonkey upgrade; this removes the
now unused annotation.

components/script/dom/bindings/codegen/CodegenRust.py

@@ -3445,7 +3445,6 @@ class CGUnionStruct(CGThing):
         # XXXManishearth The following should be #[must_root],
         # however we currently allow it till #2661 is fixed
         return ("""\
-#[allow(unrooted_must_root)]
 pub enum %s {
 %s
 }