remove reference to unaccepted RFC

Author: RalfJung

src/items/unions.md

@@ -145,9 +145,6 @@ aspects of Rust language (such as privacy, name resolution, type inference,
 generics, trait implementations, inherent implementations, coherence, pattern
 checking, etc etc etc).
 
-More detailed specification for unions, including unstable bits, can be found
-in [RFC 1897 "Unions v1.2"](https://github.com/rust-lang/rfcs/pull/1897).
-
 [IDENTIFIER]: identifiers.html
 [_Generics_]: items/generics.html
 [_WhereClause_]: items/generics.html#where-clauses