Remove "disk_image_format" config, since it isn't used. [closes GH-478]

config/default.rb

@@ -19,7 +19,6 @@ Vagrant::Config.run do |config|
   config.vm.box_url = nil
   config.vm.base_mac = nil
   config.vm.forward_port("ssh", 22, 2222, :auto => true)
-  config.vm.disk_image_format = 'VMDK'
   config.vm.boot_mode = "vrdp"
   config.vm.system = :linux
 

lib/vagrant/config/vm.rb

@@ -19,7 +19,6 @@ module Vagrant
       attr_reader :shared_folders
       attr_reader :network_options
       attr_reader :provisioners
-      attr_accessor :disk_image_format
       attr_accessor :system
 
       def initialize