Remove unused feature gates from librustc_interface

Author: bjorn3

src/librustc_interface/lib.rs

@@ -2,10 +2,8 @@
 #![feature(box_syntax)]
 #![feature(set_stdio)]
 #![feature(nll)]
-#![feature(arbitrary_self_types)]
 #![feature(generator_trait)]
 #![feature(generators)]
-#![cfg_attr(unix, feature(libc))]
 #![recursion_limit = "256"]
 
 #[cfg(unix)]