Rollup merge of #85571 - workingjubilee:reverse-prepend, r=Amanieu

Remove surplus prepend LinkedList fn

This nightly library feature provides a function on `LinkedList<T>` that is identical to `fn append` with a reversed order of arguments. Observe this diff against the `fn append` doctest:
```diff
+#![feature(linked_list_prepend)]
 fn main() {
    use std::collections::LinkedList;
    let mut list1 = LinkedList::new();
    list1.push_back('a');
    let mut list2 = LinkedList::new();
    list2.push_back('b');
    list2.push_back('c');

-    list1.append(&mut list2);
+    list2.prepend(&mut list1);

-    let mut iter = list1.iter();
+    let mut iter = list2.iter();
     assert_eq!(iter.next(), Some(&'a'));
     assert_eq!(iter.next(), Some(&'b'));
     assert_eq!(iter.next(), Some(&'c'));
     assert!(iter.next().is_none());

-    assert!(list2.is_empty());
+    assert!(list1.is_empty());
 }
```

As this has received no obvious request to stabilize it, nor does it have a tracking issue, and was left on nightly and the consensus seems to have been to deprecate it in this pre-1.0 PR in 2014, https://github.com/rust-lang/rust/pull/20356, I propose simply removing it.

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