servo: Merge #18601 - Remove unused 'untagged_unions' feature gate (from mbrubeck:features); r=KiChjang

CC #5286

- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes do not require tests because _____

Source-Repo: https://github.com/servo/servo
Source-Revision: 352b129dc5b237757ade67448cc42f327780417e

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 0162983ceffbaeb3221c38f8b7319033694eaaef

servo/components/script/lib.rs

@@ -15,7 +15,6 @@
 #![feature(plugin)]
 #![feature(proc_macro)]
 #![feature(try_from)]
-#![feature(untagged_unions)]
 
 #![deny(unsafe_code)]
 #![allow(non_snake_case)]