Deprecating brew cask plugin (#5191)

The good completion is now part of Homebrew so this one  will always be outdated.

See https://github.com/Homebrew/brew/pull/407
and https://github.com/Homebrew/brew/pull/936.

plugins/brew-cask/brew-cask.plugin.zsh

@@ -1,84 +0,0 @@
-# Autocompletion for homebrew-cask.
-#
-# This script intercepts calls to the brew plugin and adds autocompletion
-# for the cask subcommand.
-#
-# Author: https://github.com/pstadler
-
-compdef _brew-cask brew
-
-_brew-cask()
-{
-  local curcontext="$curcontext" state line
-  typeset -A opt_args
-
-  _arguments -C \
-    ':command:->command' \
-    ':subcmd:->subcmd' \
-    '*::options:->options'
-
-  case $state in
-    (command)
-      __call_original_brew
-      cask_commands=(
-        'cask:manage casks'
-      )
-      _describe -t commands 'brew cask command' cask_commands ;;
-
-    (subcmd)
-      case "$line[1]" in
-        cask)
-          if (( CURRENT == 3 )); then
-            local -a subcommands
-            subcommands=(
-              "alfred:used to modify Alfred's scope to include the Caskroom"
-              'audit:verifies installability of casks'
-              'checklinks:checks for bad cask links'
-              'cleanup:cleans up cached downloads'
-              'create:creates a cask of the given name and opens it in an editor'
-              'doctor:checks for configuration issues'
-              'edit:edits the cask of the given name'
-              'fetch:downloads Cask resources to local cache'
-              'home:opens the homepage of the cask of the given name'
-              'info:displays information about the cask of the given name'
-              'install:installs the cask of the given name'
-              'list:with no args, lists installed casks; given installed casks, lists installed files'
-              'search:searches all known casks'
-              'uninstall:uninstalls the cask of the given name'
-              "update:a synonym for 'brew update'"
-            )
-            _describe -t commands "brew cask subcommand" subcommands
-          fi ;;
-
-        *)
-          __call_original_brew ;;
-      esac ;;
-
-    (options)
-      local -a casks installed_casks
-      local expl
-      case "$line[2]" in
-        list|uninstall)
-          __brew_installed_casks
-          _wanted installed_casks expl 'installed casks' compadd -a installed_casks ;;
-        audit|edit|home|info|install)
-          __brew_all_casks
-          _wanted casks expl 'all casks' compadd -a casks ;;
-      esac ;;
-  esac
-}
-
-__brew_all_casks() {
-  casks=(`brew cask search`)
-}
-
-__brew_installed_casks() {
-  installed_casks=(`brew cask list`)
-}
-
-__call_original_brew()
-{
-  local ret=1
-  _call_function ret _brew
-  compdef _brew-cask brew
-}