remove some no-longer-needed rustc_allow_const_fn_unstable

Author: RalfJung

library/core/src/mem/maybe_uninit.rs

@@ -390,9 +390,6 @@ impl<T> MaybeUninit<T> {
     #[must_use]
     #[rustc_diagnostic_item = "maybe_uninit_zeroed"]
     #[stable(feature = "maybe_uninit", since = "1.36.0")]
-    // These are OK to allow since we do not leak &mut to user-visible API
-    #[rustc_allow_const_fn_unstable(const_mut_refs)]
-    #[rustc_allow_const_fn_unstable(const_ptr_write)]
     #[rustc_const_stable(feature = "const_maybe_uninit_zeroed", since = "1.75.0")]
     pub const fn zeroed() -> MaybeUninit<T> {
         let mut u = MaybeUninit::<T>::uninit();

library/core/src/ptr/mod.rs

@@ -844,7 +844,6 @@ pub const fn from_ref<T: ?Sized>(r: &T) -> *const T {
 #[must_use]
 #[stable(feature = "ptr_from_ref", since = "1.76.0")]
 #[rustc_const_stable(feature = "ptr_from_ref", since = "1.76.0")]
-#[rustc_allow_const_fn_unstable(const_mut_refs)]
 #[rustc_never_returns_null_ptr]
 pub const fn from_mut<T: ?Sized>(r: &mut T) -> *mut T {
     r

library/core/src/slice/mod.rs

@@ -764,7 +764,6 @@ impl<T> [T] {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_stable(feature = "const_ptr_offset", since = "1.61.0")]
-    #[rustc_allow_const_fn_unstable(const_mut_refs)]
     #[rustc_never_returns_null_ptr]
     #[inline(always)]
     #[must_use]