Remove implied rust_2018 feature

We are already on the edition and this feature is implied.

src/lib.rs

@@ -1,6 +1,5 @@
 // error-pattern:cargo-clippy
 #![feature(plugin_registrar)]
-#![feature(rust_2018_preview)]
 #![feature(rustc_private)]
 #![feature(macro_vis_matcher)]
 #![allow(unknown_lints)]