Do not `brew install brew-cask`

Refs caskroom/homebrew-cask#15381

Closes Homebrew/homebrew#46845.

Library/Homebrew/cmd/install.rb

@@ -25,8 +25,6 @@ module Homebrew
       formulae = []
 
       if ARGV.casks.any?
-        brew_cask = Formulary.factory("brew-cask")
-        install_formula(brew_cask) unless brew_cask.installed?
         args = []
         args << "--force" if ARGV.force?
         args << "--debug" if ARGV.debug?

share/doc/homebrew/External-Commands.md

@@ -73,7 +73,6 @@ These commands have been contributed by Homebrew users but are not included in t
 > Install using:
 > ```
   $ brew tap caskroom/cask
-  $ brew install brew-cask
 > ```
 
 ### brew-gem