Remove $download_id from edd_show_has_purchased_item_message()

Removed $download_id from edd_show_has_purchased_item_message() function and passed in get_the_ID() since $download_id will always be the post ID anyway 

This now makes it possible to remove and rehook the message without needing to pass $post->ID to the action hook, as this generates `PHP Notice:  Trying to get property of non-object` errors on non download pages.

Keeps it cleaner, all in favour?