Remove now unused lang_items feature in IO primitives (#4596)

We don't use any explicit `#[lang = ...]` items anymore (`panic_fmt`
is replaced with now-stable `#[panic_implementation]`, OOM is handled
via `alloc_error_handler` directly etc.), so I thought it'd be good to
remove it for clarity to limit used nightly features.

substrate/primitives/io/src/lib.rs

@@ -19,7 +19,6 @@
 #![warn(missing_docs)]
 
 #![cfg_attr(not(feature = "std"), no_std)]
-#![cfg_attr(not(feature = "std"), feature(lang_items))]
 #![cfg_attr(not(feature = "std"), feature(alloc_error_handler))]
 #![cfg_attr(not(feature = "std"), feature(core_intrinsics))]