Remove $download_id from edd_show_has_purchased_item_message()

Removed $download_id from edd_show_has_purchased_item_message() function since it will always be the post ID. This makes it possible to remove and rehook the message without passing $post->ID to the action hook.
swapped get_the_ID() for $post->ID to avoid PHP notice
Merge pull request #1230 from sumobi/master

Remove $download_id from edd_show_has_purchased_item_message()