Remove unnecessary #[unstable] from internal macro.

After this change, all library #![feature]s enabled in core are for
const fns.

library/core/src/internal_macros.rs

@@ -77,7 +77,6 @@ macro_rules! forward_ref_op_assign {
 }
 
 /// Create a zero-size type similar to a closure type, but named.
-#[unstable(feature = "std_internals", issue = "none")]
 macro_rules! impl_fn_for_zst {
     ($(
         $( #[$attr: meta] )*

library/core/src/lib.rs

@@ -70,7 +70,7 @@
 #![warn(missing_docs)]
 #![allow(explicit_outlives_requirements)]
 
-// Library features:
+// Library features for const fns:
 #![feature(const_align_of_val)]
 #![feature(const_alloc_layout)]
 #![feature(const_arguments_as_str)]
@@ -105,7 +105,6 @@
 #![feature(duration_consts_2)]
 #![feature(ptr_metadata)]
 #![feature(slice_ptr_get)]
-#![feature(std_internals)]
 #![feature(variant_count)]
 
 // Language features: