Revert "Implement Index[Mut] for arrays"

This reverts commit c03dfa6.

The commit broke code that worked on stable and beta:

const fn foo() {
    let mut array = [[0; 1]; 1];
    array[0][0] = 1;
}

But this was not noticed until the commit landed.

Fixes rust-lang#79152

library/core/src/array/mod.rs

@@ -12,7 +12,6 @@ use crate::convert::{Infallible, TryFrom};
 use crate::fmt;
 use crate::hash::{self, Hash};
 use crate::marker::Unsize;
-use crate::ops::{Index, IndexMut};
 use crate::slice::{Iter, IterMut};
 
 mod iter;
@@ -209,30 +208,6 @@ impl<'a, T, const N: usize> IntoIterator for &'a mut [T; N] {
     }
 }
 
-#[stable(feature = "index_trait_on_arrays", since = "1.50.0")]
-impl<T, I, const N: usize> Index<I> for [T; N]
-where
-    [T]: Index<I>,
-{
-    type Output = <[T] as Index<I>>::Output;
-
-    #[inline]
-    fn index(&self, index: I) -> &Self::Output {
-        Index::index(self as &[T], index)
-    }
-}
-
-#[stable(feature = "index_trait_on_arrays", since = "1.50.0")]
-impl<T, I, const N: usize> IndexMut<I> for [T; N]
-where
-    [T]: IndexMut<I>,
-{
-    #[inline]
-    fn index_mut(&mut self, index: I) -> &mut Self::Output {
-        IndexMut::index_mut(self as &mut [T], index)
-    }
-}
-
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<A, B, const N: usize> PartialEq<[B; N]> for [A; N]
 where