The following two lines aren't needed in the make file as these are the default values.
libraries[flexslider][directory_name] = "flexslider"
libraries[flexslider][type] = "library"
Drush Make will know that it's a library because of "libraries", and the next part ("flexslider") determines the name of the directory that it's put into. There's no advantage to defining these again, so these lines might as well be removed.