Remove search from brew-cask man page

[gguillotte]

brew cask no longer supports the search command, which was removed from
the homebrew/brew-cask documentation. However, it's still in the
brew repo's man page for brew-cask.

Remove search from the man page.

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• [N/A] Have you written new tests for your changes? Here's an example.
• [N/A] Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?

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[MikeMcQuaid]

Thanks @gguillotte!