servo: Merge #6809 - Remove unrooted_must_root annotation from unions (fixes #2661) (from Ms2ger:unrooted_must_root); r=metajack

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

Source-Repo: https://github.com/servo/servo
Source-Revision: 7adc336aaacfb5e544a9c8c4e0387afa457e5d72

servo/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
 }