Remove vec_linked_list.

It provides a way to effectively embed a linked list within an
`IndexVec` and also iterate over that list. It's written in a very
generic way, involving two traits `Links` and `LinkElem`. But the
`Links` trait is only impl'd for `IndexVec` and `&IndexVec`, and the
whole thing is only used in one module within `rustc_borrowck`. So I
think it's over-engineered and hard to read. Plus it has no comments.

This commit removes it, and adds a (non-generic) local iterator for the
use within `rustc_borrowck`. Much simpler.

compiler/rustc_data_structures/src/lib.rs

@@ -85,7 +85,6 @@ pub mod temp_dir;
 pub mod transitive_relation;
 pub mod unhash;
 pub mod unord;
-pub mod vec_linked_list;
 pub mod work_queue;
 
 mod atomic_ref;