fix tests

Author: RalfJung

src/test/codegen/box-maybe-uninit.rs

@@ -1,6 +1,5 @@
 // compile-flags: -O
 #![crate_type="lib"]
-#![feature(maybe_uninit)]
 
 use std::mem::MaybeUninit;
 

src/test/run-pass/panic-uninitialized-zeroed.rs

@@ -2,7 +2,7 @@
 // This test checks that instantiating an uninhabited type via `mem::{uninitialized,zeroed}` results
 // in a runtime panic.
 
-#![feature(never_type, maybe_uninit)]
+#![feature(never_type)]
 
 use std::{mem, panic};