Remove default value from Keg#link_dir mode parameter

Now that this is always called with an explicit mode argument, let's
make sure it stays that way.

Library/Homebrew/keg.rb

@@ -255,7 +255,7 @@ class Keg < Pathname
   end
 
   # symlinks the contents of self+foo recursively into #{HOMEBREW_PREFIX}/foo
-  def link_dir foo, mode=OpenStruct.new
+  def link_dir foo, mode
     root = self+foo
     return unless root.exist?
     root.find do |src|