Remove lint annotations in specific crates that are already enforced by rustbuild

Remove some random unnecessary lint `allow`s

src/librustc_errors/lib.rs

@@ -5,11 +5,9 @@
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/")]
 
 #![feature(crate_visibility_modifier)]
-#![allow(unused_attributes)]
 #![cfg_attr(unix, feature(libc))]
 #![feature(nll)]
 #![feature(optin_builtin_traits)]
-#![deny(rust_2018_idioms)]
 #![deny(unused_lifetimes)]
 
 pub use emitter::ColorConfig;