Remove surplus prepend LinkedList fn

[workingjubilee]

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:

+#![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, #20356, I propose simply removing it.

[rust-highfive]

r? @Mark-Simulacrum

(rust-highfive has picked a reviewer for you, use r? to override)

[Mark-Simulacrum]

I don't necessarily mind removing this, though it might make sense to deprecate it for a bit so that the transition path can be signaled via the deprecation message.

r? @dtolnay

[dtolnay]

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, #20356, I propose simply removing it.

This is not the method from #20356. That one was deleted in #20462. The method in this PR is from #68123.

r? @Amanieu

[workingjubilee]

I am learning so many things about the history of this source file today.

[Amanieu]

I am not sure how that method got in there, it has nothing to do with cursors.

@bors r+

[bors]

📌 Commit c516e71 has been approved by Amanieu