trait_upcasting is not currently an incomplete feature.

Author: kpreid

src/doc/unstable-book/src/language-features/trait-upcasting.md

@@ -12,7 +12,6 @@ so long as `Bar: Foo`.
 
 ```rust,edition2018
 #![feature(trait_upcasting)]
-#![allow(incomplete_features)]
 
 trait Foo {}