Formula/brew-cask.rb: remove old Cask formula. (#23890)

It's no longer needed as it's been an internal Homebrew command for
quite a while at this point.

Formula/.rubocop.yml

@@ -1,5 +0,0 @@
-inherit_from:
-  - ../.rubocop.yml
-
-Style/HashSyntax:
-  EnforcedStyle: hash_rockets

Formula/brew-cask.rb

@@ -1,24 +0,0 @@
-class BrewCask < Formula
-  homepage "https://github.com/caskroom/homebrew-cask/"
-  url "https://github.com/caskroom/homebrew-cask.git", :tag => "v0.60.1"
-
-  depends_on :ruby => "2.0"
-
-  UNINSTALL_MSG = <<-EOS.undent
-    You must uninstall this formula. It is no longer needed to stay up to date,
-    as Homebrew now takes care of that automatically.
-  EOS
-
-  def install
-    (buildpath / "UPGRADE").write UNINSTALL_MSG
-    prefix.install "UPGRADE"
-  end
-
-  def caveats
-    UNINSTALL_MSG
-  end
-
-  test do
-    system "brew", "cask", "info", "google-chrome"
-  end
-end