Remove unused function from bump-cask-pr

[apainintheneck]

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The function fetch_cask was originally added in f65f0f9 and the final reference to it was removed in 092095a.

[BrewTestBot]

Review period will end on 2022-09-29 at 04:27:55 UTC.

[BrewTestBot]

Review period skipped due to critical label.