Remove unnecessary forward_inner_docs hack

and replace it with `extended_key_value_attributes` feature.

compiler/rustc_errors/src/lib.rs

@@ -5,6 +5,7 @@
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![feature(crate_visibility_modifier)]
 #![feature(backtrace)]
+#![feature(extended_key_value_attributes)]
 #![feature(nll)]
 
 #[macro_use]