Remove deprecated and unstable slice_partition_at_index functions

They have been deprecated since commit 01ac5a97c9
which was part of the 1.49.0 release, so from the point of nightly,
11 releases ago.

library/core/src/slice/mod.rs

@@ -2558,50 +2558,6 @@ impl<T> [T] {
         sort::quicksort(self, |a, b| f(a).lt(&f(b)));
     }
 
-    /// Reorder the slice such that the element at `index` is at its final sorted position.
-    #[unstable(feature = "slice_partition_at_index", issue = "55300")]
-    #[rustc_deprecated(since = "1.49.0", reason = "use the select_nth_unstable() instead")]
-    #[inline]
-    pub fn partition_at_index(&mut self, index: usize) -> (&mut [T], &mut T, &mut [T])
-    where
-        T: Ord,
-    {
-        self.select_nth_unstable(index)
-    }
-
-    /// Reorder the slice with a comparator function such that the element at `index` is at its
-    /// final sorted position.
-    #[unstable(feature = "slice_partition_at_index", issue = "55300")]
-    #[rustc_deprecated(since = "1.49.0", reason = "use select_nth_unstable_by() instead")]
-    #[inline]
-    pub fn partition_at_index_by<F>(
-        &mut self,
-        index: usize,
-        compare: F,
-    ) -> (&mut [T], &mut T, &mut [T])
-    where
-        F: FnMut(&T, &T) -> Ordering,
-    {
-        self.select_nth_unstable_by(index, compare)
-    }
-
-    /// Reorder the slice with a key extraction function such that the element at `index` is at its
-    /// final sorted position.
-    #[unstable(feature = "slice_partition_at_index", issue = "55300")]
-    #[rustc_deprecated(since = "1.49.0", reason = "use the select_nth_unstable_by_key() instead")]
-    #[inline]
-    pub fn partition_at_index_by_key<K, F>(
-        &mut self,
-        index: usize,
-        f: F,
-    ) -> (&mut [T], &mut T, &mut [T])
-    where
-        F: FnMut(&T) -> K,
-        K: Ord,
-    {
-        self.select_nth_unstable_by_key(index, f)
-    }
-
     /// Reorder the slice such that the element at `index` is at its final sorted position.
     ///
     /// This reordering has the additional property that any value at position `i < index` will be

library/core/tests/lib.rs

@@ -46,7 +46,6 @@
 #![feature(is_sorted)]
 #![feature(pattern)]
 #![feature(sort_internals)]
-#![feature(slice_partition_at_index)]
 #![feature(slice_take)]
 #![feature(maybe_uninit_uninit_array)]
 #![feature(maybe_uninit_array_assume_init)]