Pulled in helper method using letfn

Decided to refactor the code to pull the unwind-list-delete function into the delete-index function using letfn since there is only one place that the function is called from. It also matches the pattern I used throughout the file.

I'm not sure if this makes the code more readable or creates a nesting depth in the definition of the function. I may want to limit the use of letfn to functions that are even short than this to break up the code for human readability.