grml-cheatcodes + isolinux splashes: drop xkeyboard + lang=*-iso boot options

The xkeyboard boot option is no longer supported as such (see
relevant changes in grml-live commit 3a054970e and grml-autoconfig
commit fc67c1849a4), so longer announce it.

The lang=*-iso is supposed to disappear and no longer supported as
such, so don't announce it either.