Drop generated highlight.nu which has no source anymore

`highlight.fish` has been removed upstream in 2cb60bed1000a781bf427b56108bba2e7c68ba32

> Remove share/completions/highlight.fish
>
> Highlight ships its own completion script:
> https://gitlab.com/saalen/highlight/-/blob/master/sh-completion/highlight.fish

fish-shell/fish-shell@2cb60be

Author: Kissaki