Remove brew-dirty

Replaced with `brew list --versions --multiple`.

Library/Contributions/cmd/brew-dirty.rb

@@ -1,8 +0,0 @@
-# See: http://github.com/Homebrew/homebrew/issues/issue/1359
-
-require 'keg'
-
-HOMEBREW_CELLAR.subdirs.each do |rack|
-  versions = rack.subdirs.map { |d| Keg.new(d).version }
-  puts "#{rack.basename} (#{versions.join(', ')})" if versions.size > 1
-end