Fixed call to delete_device (method name changed)

Fixed a call to delete_device method (nexpose-0.5.4.gem) which was generating this error:

rm_all_assets_from_group.rb:49:in `block in <main>': undefined method `device_delete' for #<Nexpose::Connection:0x00000002da2328> (NoMethodError)
        from rm_all_assets_from_group.rb:45:in `each'
        from rm_all_assets_from_group.rb:45:in `<main>'