If i'm not wrong, it is legal to remove. But i believe that the developers would appreaciate you not removing it and helping them pass the word of their hardwork on.
According to the GPL (which the product is licensed under), removing the copyright is legal. But as they say, near the copyright, they won't give you ANY support. Watch out for this on some other boards such as MyBB which has a policy against this. Always read the license before proceeding.