Remove warning

Author: Manishearth

clippy_lints/src/lib.rs

@@ -13,7 +13,6 @@
 #![feature(macro_at_most_once_rep)]
 #![feature(rust_2018_preview)]
 #![warn(rust_2018_idioms)]
-#![allow(macro_use_extern_crate)]
 
 use toml;
 use rustc_plugin;