formulae: remove unnecessary crate livecheck blocks

[samford]

• Have you followed the guidelines for contributing?
• Have you ensured that your commits follow the commit style guide?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
• Does your build pass brew audit --strict <formula> (after doing HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>)? If this is a new formula, does it pass brew audit --new <formula>?

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We recently added a Crate strategy to livecheck (Homebrew/brew#16620), which uses similar logic to the pngquant livecheck block. We can now remove the livecheck blocks for oakc and pngquant, as the Crate strategy will work for these formulae by default.