I'm unfortunately not trained in legal matters well enough to know what the distinction is or if it even matters. The GNU license doesn't like copyrights removed, but don't know how specific it is. So I can't give a solid answer one way or another here. I'll make a note to ask this the next time I come across someone that knows about this stuff though. However, outside of this, there are a couple reasons you may not want to remove it: 1) unless you make your own admin theme, you'll have to do the same thing every time you upgrade (which would be a pain); 2) With GNU software, it's required that the client gets the same GNU license, and they should understand it and that it's copyrighted software. You don't want them to think they are paying you for the software itself rather than your services.