brew: remove duplicated and outdated brew completion script

Homebrew has began to ship its own official zsh completion script.
So we could remove the duplicated and outdated version from OMZ.

plugins/brew/_brew

@@ -1,145 +0,0 @@
-#compdef brew
-#autoload
-
-# imported from https://github.com/Homebrew/homebrew/blob/29f73d2212c2b202fe25f69dcbf440d8391fa4c9/Library/Contributions/brew_zsh_completion.zsh
-
-# Brew ZSH completion function
-# Drop this somewhere in your $fpath (like /usr/share/zsh/site-functions)
-# and rename it _brew
-#
-# altered from _fink
-
-_brew_all_formulae() {
-  formulae=(`brew search`)
-}
-
-_brew_installed_formulae() {
-  installed_formulae=(`brew list`)
-}
-
-_brew_installed_taps() {
-  installed_taps=(`brew tap`)
-}
-
-_brew_official_taps() {
-  official_taps=(`brew tap --list-official`)
-}
-
-_brew_pinned_taps() {
-  pinned_taps=(`brew tap --list-pinned`)
-}
-
-_brew_outdated_formulae() {
-  outdated_formulae=(`brew outdated`)
-}
-
-local -a _1st_arguments
-_1st_arguments=(
-  'audit:check formulae for Homebrew coding style'
-  'cat:display formula file for a formula'
-  'cleanup:uninstall unused and old versions of packages'
-  'commands:show a list of commands'
-  'config:show homebrew and system configuration'
-  'create:create a new formula'
-  'deps:list dependencies and dependants of a formula'
-  'desc:display a description of a formula'
-  'doctor:audits your installation for common issues'
-  'edit:edit a formula'
-  'fetch:download formula resources to the cache'
-  'gist-logs:generate a gist of the full build logs'
-  'home:visit the homepage of a formula or the brew project'
-  'info:information about a formula'
-  'install:install a formula'
-  'reinstall:install a formula anew; re-using its current options'
-  'leaves:show installed formulae that are not dependencies of another installed formula'
-  'link:link a formula'
-  'linkapps:symlink .app bundles provided by formulae into /Applications'
-  'list:list files in a formula or not-installed formulae'
-  'log:git commit log for a formula'
-  'missing:check all installed formuale for missing dependencies.'
-  'migrate:migrate renamed formula to new name'
-  'outdated:list formulae for which a newer version is available'
-  'pin:pin specified formulae'
-  'postinstall:perform post_install for a given formula'
-  'prune:remove dead links'
-  'remove:remove a formula'
-  'search:search for a formula (/regex/ or string)'
-  'switch:switch between different versions of a formula'
-  'tap:tap a new formula repository from GitHub, or list existing taps'
-  'tap-info:information about a tap'
-  'tap-pin:pin a tap'
-  'tap-unpin:unpin a tap'
-  'test-bot:test a formula and build a bottle'
-  'uninstall:uninstall a formula'
-  'unlink:unlink a formula'
-  'unlinkapps:remove symlinked .app bundles provided by formulae from /Applications'
-  'unpin:unpin specified formulae'
-  'untap:remove a tapped repository'
-  'update:fetch latest version of Homebrew and all formulae'
-  'upgrade:upgrade outdated formulae'
-  'uses:show formulae which depend on a formula'
-  `brew commands --quiet --include-aliases`
-)
-
-local expl
-local -a formulae installed_formulae installed_taps official_taps outdated_formulae
-
-_arguments \
-  '(-v)-v[verbose]' \
-  '(--cellar)--cellar[brew cellar]' \
-  '(--env)--env[brew environment]' \
-  '(--repository)--repository[brew repository]' \
-  '(--version)--version[version information]' \
-  '(--prefix)--prefix[where brew lives on this system]' \
-  '(--cache)--cache[brew cache]' \
-  '*:: :->subcmds' && return 0
-
-if (( CURRENT == 1 )); then
-  _describe -t commands "brew subcommand" _1st_arguments
-  return
-fi
-
-case "$words[1]" in
-  install|reinstall|audit|home|homepage|log|info|abv|uses|cat|deps|desc|edit|options|switch)
-    _brew_all_formulae
-    _wanted formulae expl 'all formulae' compadd -a formulae ;;
-  linkapps|unlinkapps)
-    _arguments \
-      '(--local)--local[operate on ~/Applications instead of /Applications]' \
-      '1: :->forms' && return 0
-
-    if [[ "$state" == forms ]]; then
-      _brew_installed_formulae
-      _wanted installed_formulae expl 'installed formulae' compadd -a installed_formulae
-    fi ;;
-  list|ls)
-    _arguments \
-      '(--unbrewed)--unbrewed[files in brew --prefix not controlled by brew]' \
-      '(--pinned)--pinned[list all versions of pinned formulae]' \
-      '(--versions)--versions[list all installed versions of a formula]' \
-      '1: :->forms' && return 0
-
-      if [[ "$state" == forms ]]; then
-        _brew_installed_formulae
-        _wanted installed_formulae expl 'installed formulae' compadd -a installed_formulae
-      fi ;;
-  remove|rm|uninstall|unlink|cleanup|link|ln|pin|unpin)
-    _brew_installed_formulae
-    _wanted installed_formulae expl 'installed formulae' compadd -a installed_formulae ;;
-  search|-S)
-    _arguments \
-      '(--macports)--macports[search the macports repository]' \
-      '(--fink)--fink[search the fink repository]' ;;
-  untap|tap-info|tap-pin)
-    _brew_installed_taps
-    _wanted installed_taps expl 'installed taps' compadd -a installed_taps ;;
-  tap)
-    _brew_official_taps
-    _wanted official_taps expl 'official taps' compadd -a official_taps ;;
-  tap-unpin)
-    _brew_pinned_taps
-    _wanted pinned_taps expl 'pinned taps' compadd -a pinned_taps ;;
-  upgrade)
-    _brew_outdated_formulae
-    _wanted outdated_formulae expl 'outdated formulae' compadd -a outdated_formulae ;;
-esac