Allow re-using Flickr photos

When you try to add an item that is already in the media library, this fails with a message "You have entered a URL for a image that is already in your library." It seems this was a temporary fix like in media_youtube #952422: Trying to add the same YouTube URL twice causes a primary key violation, but re-using items is now possible as in #1213184: Allow YouTube videos to be re-used. Looks like it's only a matter of removing the validation code?