Revert "Put slicing syntax behind a feature gate."

This reverts commit 95cfc35607.

src/libcore/lib.rs

@@ -57,9 +57,8 @@
        html_playground_url = "http://play.rust-lang.org/")]
 
 #![no_std]
-#![allow(unknown_features)]
 #![feature(globs, intrinsics, lang_items, macro_rules, managed_boxes, phase)]
-#![feature(simd, unsafe_destructor, slicing_syntax)]
+#![feature(simd, unsafe_destructor)]
 #![deny(missing_doc)]
 
 mod macros;