Remove unnecessary feature gates from const fns

Author: oli-obk

src/libstd/lib.rs

@@ -272,6 +272,7 @@
 #![feature(libc)]
 #![feature(link_args)]
 #![feature(linkage)]
+#![feature(min_const_unsafe_fn)]
 #![feature(needs_panic_runtime)]
 #![feature(never_type)]
 #![feature(nll)]