Remove dependency on linked-list crate

[pemistahl]

grex uses a dependency on the linked-list crate whose last version was released nearly five years ago. I once added this dependency because it provides a cursor implementation of a linked list that allows to modify the list during iteration. This comes in handy for the implementation of the DFA minimization algorithm.

However, as it turns out, it is not so difficult to rewrite the algorithm using a simple vector instead of the cursor-based linked list. So it makes sense to get rid of the no longer maintained dependency.