extend/os: remove cask install code

This code is unnecessary since `--formula` is passed
by default to the install command on Linux making it
impossible to reach `Cask::Cmd::Install.install_casks`
since it is surrounded by `if casks.any?; end`.

Library/Homebrew/cask/cmd/install.rb

@@ -2,7 +2,6 @@
 # frozen_string_literal: true
 
 require "cask_dependent"
-require "extend/os/cask/cmd/install"
 
 module Cask
   class Cmd

Library/Homebrew/extend/os/cask/cmd/install.rb

@@ -1,4 +0,0 @@
-# typed: strict
-# frozen_string_literal: true
-
-require "extend/os/linux/cask/cmd/install" if OS.linux?

Library/Homebrew/extend/os/linux/cask/cmd/install.rb

@@ -1,29 +0,0 @@
-# typed: false
-# frozen_string_literal: true
-
-require "cask_dependent"
-
-module Cask
-  class Cmd
-    # Cask implementation of the `brew install` command.
-    #
-    # @api private
-    class Install < AbstractCommand
-      def self.install_casks(
-        _casks,
-        verbose: nil,
-        force: nil,
-        adopt: nil,
-        binaries: nil,
-        skip_cask_deps: nil,
-        require_sha: nil,
-        quarantine: nil,
-        quiet: nil,
-        zap: nil,
-        dry_run: nil
-      )
-        odie "Installing casks is supported only on macOS"
-      end
-    end
-  end
-end