Remove useless `fill_in` function
Remove useless `fill_in` function

It was only used once, in a function that was otherwise trivial.

aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust