An option to easily delete a layer from a group song isn’t available currently. There is however an unofficial trick: You can “hide” a group song you joined from your profile. That will remove the entry from your profile on iOS devices. If there is another join after the song was hidden (you can join silently yourself to achieve this), ALL layers of the person who has hidden the song will usually be removed. Only the person who created this layer can use this trick though. The person who opened the song cannot remove unwanted layers or their own.