Remove unnecessary feature(cfg_if)

Author: gnzlbg

src/libcore/tests/cfg_if_override.rs

@@ -1,6 +1,5 @@
 //! Test that overriding cfg_if with our own cfg_if macro does not break
 //! anything.
-#![feature(cfg_if)]
 
 macro_rules! cfg_if {
     (()) => {