Replace the usage of outdated brew cask list in zsh completion

[Akylzhan]

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Fixes #8640

[MikeMcQuaid]

Should __brew_cask helper function removed then? It's not used anymore.

Yes, I think so 👍🏻

Also, I wasn't sure what to do about the sed filter, so that's why I didn't create a PR. I guess it is not needed?

If it works without: yeh, can be left as-is.

[Akylzhan]

@MikeMcQuaid Should I delete another functions that aren't used? Like _brew_cask_audit, _brew_cask_cat (which is duplicated), and others?

[MikeMcQuaid]

@Akylzhan yes, please!

[MikeMcQuaid]

Thanks again for your PR @Akylzhan, you rock!