cask: remove old prefix placeholder

[apainintheneck]

This was just a temporary workaround while we
waited for the API to regenerate the JSON content.

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This is just a follow-up to changing the prefix placeholder for casks in #14615. It should be fine now since it's been around a month since the original PR and the API JSON gets reloaded for users every day or so. There is an edge case where someone is on the most recent brew version but has a really old cask.json file but I doubt that's worth worrying about.

[BrewTestBot]

Review period will end on 2023-03-21 at 00:00:00 UTC.

[issyl0]

I saw this earlier and wondered when the TODO was written. Good cleanup!

[MikeMcQuaid]

Thanks for the cleanup and sorry for missing this @apainintheneck!