Remove unused feature gate from librustc_lint

Author: bjorn3

src/librustc_lint/lib.rs

@@ -28,7 +28,6 @@
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/")]
 #![cfg_attr(test, feature(test))]
 #![feature(bool_to_option)]
-#![feature(box_patterns)]
 #![feature(box_syntax)]
 #![feature(crate_visibility_modifier)]
 #![feature(never_type)]