Extra icon by share button
Unfortunately, I cannot use the comment system yet, but I know what you're referring to.  Once you access the "share" feature, it automatically provides a "shortcut" to the last app you tried to "share" with.  I actually find it a pretty convenient feature.  I don't think you can get rid of it (unless you find a custom gallery or something).  If you share your photos with anybody, I'd use the most-oft used app to share with, and just leave it there. i.e. messaging / facebook / etc.